Sky, Bond - what I did was use a hand held digital metronome to find out the tempo of each of the paces, then got music to match. http://www.equimusic.com/ is a great place to find music for your horse's tempo
Thankfully the only thing he hated was the guy who pulled up with heavy rock going!
It's usually only loud enough from my pocket for me to hear, I like having a beat to work to same as when I run (didn't think he'd be too impressed with my running soundtrack! )