I don't listen to music while raiding, and especially not while PvPing. I need the audio cues to establish whether my spells are coming off, whether I'm critting (restoration shaman make a funny little noise when that happens), whether certain effects are landing, whether that warrior is Bladestorming, and so on. Of course there are visual clues as well, and for many people they will be sufficient, but I often find that, particularly in the Arena, I need all the help I can get! Also, while I do my best to keep track of every player at all times, in a 5v5 match, it's quite feasible that I can't keep my camera pointing at all 10 people, so the audio helps a great deal. I also find that music or TV can be distracting.
This is, of course, not leveled as a criticism to those who do play with music in the background, as ever with gaming is a case of diff'rent strokes for diff'rent folks. I can completely understand the use of music as a motivational tool or to amp yourself up for something challenging or to calm yourself down when things aren't going as you'd hoped.