My "riding theme song" changes from gait to gait and horse to horse.
Different rhythms fit different horses and different movements. Holding something steady like a song in your head can also help your horse find a balanced rhythm, on top of relaxing you and helping you move with, not fight against, the horse's natural rhythm.
I like soundtrack music, for some reason... This really captures Scout's big swinging "We're goin' on a trail ride!!" walk.