I love to listen music while riding. For me it doesn't matter what kind. What is helpful when I have my earphones on is the fact that I don't hear any sounds which might scare my horse. He notices that I dont react on the sound so he stays calm. Im not sure if its professional but it works.
I think it’s a way to break up the monotony and it help me keep a good rhythm. You can use jog.fm to find songs to match the gaits and make playlists. This site is for running/working out but I find it has a much wider selection of songs to use. Equimusic is another one.