I've come to the opinion that music is an inherently social art. I also think it is among the first, if not the first one humans developed/practiced - possibly even before we started breaking things up into phonemes and syllables and words. I've been out of the formal field of anthropology for years, so these thoughts aren't backed up by any citations, just observation and reasoning and that horror of horrors, personal anecdotal evidence, being somewhat acquainted with music myself.
First I need to say that it is obviously possible to enjoy music alone; let's not be specious for the sake of contention. I'm seriously interested in this, because I can't think of any other art forms where the experience is enhanced by being practiced socially. Lately I've seen artist jams, it's true, where a group gets together and draws, but that's usually a fund raiser; music is a group production normally, with solos being the rarity.
On the other hand, those examples are of performance, and most music is written by one person. Ok, and I imagine there are probably as many lyrics/music teams as there are collaboratively written types of literature. However, and (this is how I got here from part 1 and the digital piracy musings): there seems to be an immediate impulse or desire upon hearing a cool piece of music to want to share or be sharing it with someone. Even a stranger at a club or concert, but better if a friend. (Then there is the impulse to dance, but I wasn't thinking about that, and this is long already.)
The other and sort of winding down thought about music and groups is that people like to listen to music in groups much more than they like to do other things in groups. If we wanted to, we could all gather in a public venue and read novels together - but we don't. Usually if you are with a group of people in a museum, you are on a tour in a foreign country not because being in a group enhances your experience of the art of the museum. But the crowd energy makes the experience better for both the performers and the audience, and it is a group phenomena - I know from experience that an empty house is harder to connect with...