Three questions about sensual desire

Actually there's nothing really wrong with sensual pleasure at all.

It is the sensual craving that is the problem and if the pleasure is not aligned with the Eightfold path (e.g. pleasure from the four efforts, Right Livelihood, etc.).

The 2nd training, concentration/absorption, is actually all about this training to be able to focus the mind through usage of the Five Faculties on a meditation object--it is a very pleasurable experience which is naturally cultivated... like a final resting place.. which is why it is called samadhi/dhyana. Learn more about these 9 buddhist jhanas.

The above can also be said about the 1st and 3rd Buddhist trainings for various reasons: there is pleasure associated with their cultivation.

Now, to answer the heart of your question:

We gain pleasure from typical things so... all that is fine, but you will have to learn the rules for these temporary pleasure. This is all the 1st Training (morality).

For example, exercise is pleasurable and healthy for the system which the Buddha has instructed us to take care of, a temple for Enlightenment. But we should not overexert ourselves.

Eating is great, but you should not overeat.

Sometimes the things you kno about certain temporary pleasures are not what you really think they are.

So yeah, by doing anything non-Buddhist you are basically taking the risk of harming yourself if you do nor arm yourself with appropriate knowledge of things.

Anyway, in summary, the answer to your question is the 1st Training (morality), which really has no end to it, even after you are a Buddha with the full regalia of powers, you are learning things or unlearning things, developing skillful means to even greater perfection.

I think the answer lies in the practical part of Buddhism, i.e. insight meditation.

When one practices insight meditation diligently one will come to see for oneself that all physical and mental phenomena are subject to the 3 signs of existence, i.e. Impermanence, Unsatisfactoriness and Not-self, i.e. the uncontrollable and ungovernable nature of phenomena.

When experiencing these signs of existence and gaining experiental knowledge about them one's outlook on reality changes. One can still enjoy wordly stuff but without attachment or clinging because one knows that the phenomena, e.g. music will come to an end sooner or later. At some point one will just not have an interest for worldly stuff because of the inherent suffering and deep instability that are build into conventional reality.

It becomes unwholesome if one sees the song or the emotions/feelings as being permanent and satisfactory and thereby develops attachment and clinging towards them. Then one is having wrong view.

Regarding metta-meditation then one develops loving-kindness towards other beings. Loving-kindness is an antidote for one of the 3 defilements, i.e. Hatred and the second hindrance, i.e. Ill-will. In this sense one is cultivating wholesome states from which wholesome actions and results can arise.

Metta-meditation belongs to samatha meditation: which does not have 'reality' as the object of medication (meaning that one cannot use samatha meditation to develop insights into how reality functions and thereby eradicate ignorance, Aviija). Instead of 'reality', one is having a 'concept' as the object of meditation, and one is trying to make that concept permanent and stable. In samatha meditation there is a risk of developing attachment towards these pleasant states.

If one is aware of that and knows experientially that all phenomena are subject to arising and ceasing then it should not be a problem.

"If music is a sensual desire , what's wrong with that?"

The sense bases are dukkha:

“Bhikkhus, the ear is impermanent. What is impermanent is suffering. What is suffering is nonself. What is nonself should be seen as it really is with correct wisdom thus: ‘This is not mine, this I am not, this is not my self.’

-- SN 35.1

The danger of sensual pleasures:

(1) “And what, bhikkhus, is the bond of sensuality? Here, someone does not understand as they really are the origin and the passing away, the gratification, the danger, and the escape in regard to sensual pleasures. When one does not understand these things as they really are, then sensual lust, sensual delight, sensual affection, sensual infatuation, sensual thirst, sensual passion, sensual attachment, and sensual craving lie deep within one in regard to sensual pleasures. This is called the bond of sensuality.

-- AN 4.10

and:

"Again, with sensual pleasures as the cause, sensual pleasures as the source, sensual pleasures as the basis, the cause being simply sensual pleasures, people indulge in misconduct of body, speech, and mind."

-- MN 13 (Bodhi trans.)

How this danger takes place:

“Bhikkhus, when one does not know and see the ear as it actually is, when one does not know and see sounds as they actually are, when one does not know and see ear-consciousness as it actually is, when one does not know and see ear-contact as it actually is, when one does not know and see as it actually is the feeling felt as pleasant or painful or neither-painful-nor-pleasant that arises with ear-contact as condition, then one is inflamed by lust for the ear, for sounds, for ear-consciousness, for ear-contact, for the feeling felt as pleasant or painful or neither-painful-nor-pleasant that arises with ear-contact as condition.

“When one abides inflamed by lust, fettered, infatuated, contemplating gratification, then the five aggregates affected by clinging are built up for oneself in the future; and one’s craving—which brings renewal of being, is accompanied by delight and lust, and delights in this and that—increases. One’s bodily and mental troubles increase, one’s bodily and mental torments increase, one’s bodily and mental fevers increase, and one experiences bodily and mental suffering.

-- MN 149

Fully understanding and abandoning delight in the senses is a crucial aspect of the path to nibbāna:

This Noble Eightfold Path is to be developed for direct knowledge of these five cords of sensual pleasure, for the full understanding of them, for their utter destruction, for their abandoning.”

-- SN 45.176

"Sensual desire is described as the first of the five hindrances. What I am a little perplexed by is the stance of Buddhism on the positive senses."

While the gratification is acknowledged...

“And what, bhikkhus, is the gratification in the case of sensual pleasures? Bhikkhus, there are these five cords of sensual pleasure. What are the five? Forms cognizable by the eye that are wished for, desired, agreeable and likeable, connected with sensual desire, and provocative of lust. Sounds cognizable by the ear ... Odours cognizable by the nose ... Flavours cognizable by the tongue ... Tangibles cognizable by the body that are wished for, desired, agreeable and likeable, connected with sensual desire, and provocative of lust. These are the five cords of sensual pleasure. Now the pleasure and joy that arise dependent on these five cords of sensual pleasure are the gratification in the case of sensual pleasures.

-- MN 13 (Bodhi trans.)

...the training requires abandoning delight on the senses, both in its method and in its culmination:

• Concentration is impaired by the presence of sensual pleasures (thus, it's one of the five hindrances).
• The sense bases are the very source of craving -- one of the main fetters to be relinquished to attain nibbāna.

"For instance, if I desire to listen to a song, is the problem the desire, or the 'emotions and feelings' the song itself brings up?"

The problem (i.e. the obstacle to nibbāna) is the desire you mention and the absorption in the "emotions and feelings" which obscures wisdom and perpetuate ignorance of the reality. More generally, the problem is the craving and the ignorance of it's functioning from the perspective of conditioned arising and the four noble truths.

Some sensual pleasures might as well trigger wholesome states, but there is a very high risk of not being able to discern what is what (as mindfulness is neglected in delight) and fall in the trap of craving. Like an alcoholic feeling thirsty seeing a bottle of water and beer and deciding to have the beer without seeing the danger in it (to use Buddho's analogy).

Since wholesome states can be developed in other ways (safer ways, like drinking water) and sensual pleasures always carry a high danger with them (like beer to an alcoholic), it is very reasonable, from the point of view of being effective and direct, to simply regard them as a treat and prescribe its immediate abandonment:

“Seeing thus, bhikkhus, the instructed noble disciple experiences revulsion towards forms, revulsion towards sounds, revulsion towards odours, revulsion towards tastes, revulsion towards tactile objects, revulsion towards mental phenomena. Experiencing revulsion, he becomes dispassionate. Through dispassion his mind is liberated. When it is liberated there comes the knowledge: ‘It’s liberated.’ He understands: ‘Destroyed is birth, the holy life has been lived, what had to be done has been done, there is no more for this state of being.’”

-- SN 35.4

"Apart from sensual 'desire', what about just sensual 'enjoyment'?"

sensual feelings are dukkha:

Whether it be pleasant or painful
Along with the neither-painful-nor-pleasant,
Both the internal and the external,
Whatever kind of feeling there is:
Having known, “This is suffering,
Perishable, disintegrating,”
Having touched and touched them, seeing their fall,
Thus one loses one’s passion for them.

-- SN 36.62

Thus:

Sāriputta: “If they were to ask me this, venerable sir, I would answer thus: ‘Friends, there are these three feelings. What three? Pleasant feeling, painful feeling, neither-painful-nor-pleasant feeling. These three feelings, friends, are impermanent; whatever is impermanent is suffering. When this was understood, delight in feelings no longer remained present in me.’ Being asked thus, venerable sir, I would answer in such a way.” -- “Good, good, Sāriputta!”

-- SN 12.32

"If I just happen to walk by an area in which music is playing, and I find myself tapping my feet, or feeling really good; is this seen as a state we must abandon?"

If nibbana (or the fruits of this abandonment) is of concern, yes. Practice tends to naturally create dispassion towards the senses. For example, upon seeing how the sounds are gross in comparison to more sublime states of happiness and pleasures, and upon noticing how mindfulness weakens when embarking in sensual pleasures, one does not find much inclination to tap the feet anymore.

"I mean, if we never desired the music in the first place and it just happens to play around us, what is the harm in seeing the beauty in it?"

There's no inherent harm in seeing beauty. You should see beauty where there's beauty, and pleasure were there's pleasure. But nurturing that sensual pleasure (craving it) and giving attention to the sign of beauty is a problem.

“It would be better, bhikkhus, for the ear faculty to be lacerated by a sharp iron stake burning, blazing, and glowing, than for one to grasp the sign through the features in a sound cognizable by the ear. For if consciousness should stand tied to gratification in the sign or in the features, and if one should die on that occasion, it is possible that one will go to one of two destinations: hell or the animal realm. Having seen this danger, I speak thus.

-- SN 235

"If someone offers us food, are we required to not enjoy our sense pleasures of it?""

In general, we are instructed to train to be equanimous towards the senses, watching out for the dangers of craving and not being infatuated by the pleasures felt, eg, by the flavors.

"Isn't even metta meditation also a sensual desire?"

Practicing metta develops pleasure, but of a wholesome kind. This pleasure is not related to the sense bases, and is regarded as sublime, unwordly. It also promotes concentration and the factors of enlightenment. Therefore, it's a practice that leads to nibbāna.

"Also, metta-meditation frequently uses people as objects of good-will and loving-kindness -- however, this is attaching sense pleasures to mental objects, which I thought are to be abandoned?"

The pleasure of metta meditation is not born from the craving for the mental objects. Its born from the mental state/attitude that are transformed using the mental objects as vehicle.