Upnover, you have to pick songs that match your horse's tempo. So watch Rowan in his freestyle, you notice how his footfalls match the beat of the music?
You need to find your horses tempo in all gaits, and pick songs of the same tempo. Of course you can alter songs slightly, but you don't want to make an extreme change to the song if you can help it. It's also not a case of just picking one song and making up a test to go with it. You'll need walk trot and canter music, and stitch them together to match your test. Also once you're competing high enough to start introducing lengthening of strides, collection, lateral work etc. you need even more music so suit these movements.
If you're on a cute little pony with tiny strides, you don't want to pick a heavy metal piece, just as if you're in a big imposing looking horse with a real march to it's work, you're not going to pick a cutsie 'fairies' score to go with it ;)