Sorry if any of this is redundant, but I have a few thoughts, being an avid music-lover and terrible musician.
When i was a student I used to study while listening to music. At different times the music either helped or hindered. I noticed it had to do with how I engaged my mind. If I kept the music more in the back of my mind I was able to focus on learning new things pretty well, though I did also notice music with words tended to more easily confuse my train of thought when it also involved words.
There is a strong relationship with rythm and endurance, which is partly why it's so popular to run to music, but I think there are deeper relationships too. Oliver Sacks has some wonderful material on the nature of music and neurological issues...for example, some folks who are very nearly catatonic might respond to music. Music appears to often tap in to more primitive portions of the brain and I have often used music throughout my short life as a way of inducing desirable states of mind; to enter into "the zone." I've been able to induce similar states without music too though and I can see the benefit, if not the necessity, for being able to achieve these kinds of mental states without the need of listening to music. I think it is the rythm of breathing or heart-beat, for example, which serves a similar function.
One interesting thought that comes to mind, and I'm not sure where I read this or even if it's true, has to do with Terry Dobson playing Death Metal (once? occasionally? I don't know) during training. I'm somewhat desensitized now, but my intitial reaction to music like that was displeasure, and I can see some value to striving for harmony under emotionally trying circumstances such as that may cause...just as I can see how listening to a highly energetic jig (Patrick Street, anyone?) might also create an interesting dynamic, or something soft like the Moonlight Sonata would also cause a very mellow dynamic. So I see the potential value as being not only how to induce a certain feeling with certain music, but also how to overcome that feeling. For example, if a song lends itself to making you sleepy, but you strive for high levels of alertness, I can see how it might
lend istelf to actually being sleepy and striving for the same effect.
Of course, a problem might be that different people respond different to a given song, but I would think that would also be something worth studying.
(now I'm going to have to read the thread and see how applicable that was...perhaps should I add this habit of mine to the "bad habits thread.")