My instructor used to allow music during monotonous workouts ie. 5000 front kicks, or just a list of 100's that goes on for a while or during a punching/bag workout. I used to have music for my kickboxing class and it makes everyone go harder, faster to the beat. But to form, if it doesn't match up and rarely does just right it can be really disconcerting. However, if the music is matched to the form, musical form, is really fun to do. Also, it is a tournament event. But slow, romantic music is not it. I saw a BB do something like that at a tournamen for form and there was no way for him to show power with those slow soft lyrics. (he was in love-their song-oh my).
I think it can also have a mind-altering effect-energating-uplifting-motivating – to work harder. But that is what music is about. My instructor only wants Christian music. While alot of Christian music has gotten better, I really don't want to kick in tempo to Amazing Grace or whatever comes on the radio. It all has to be prescreened so the tempo is right. Not a mixed tempo, there are some good exercise tapes/cd's for cardio, kickboxing type workouts. Warmup, slow, mid-tempo and then faster for the cardio workout etc. Music is on my wish list. So, if I ever get my women's class together, I definitely will have it part of the time. Probably not during sparring either, as I would want everyone focused, concentrating and not their mind on the tune.