Cool idea! Nice visuals.
About sound: i guess it's petter to have no music then music that repeats every 2 seconds :)
i tell what i use: https://incompetech.com/music/royalty-free/music.html
thousands free tracks by Kevin MacLeod
great stuff, has sorting by Genre, mood, speed, instrument, i use it all the time. And i have 1-2 favourite tracks that i know i can throw in even if i didn't chose anything, and it will just work fine :)
Great job, good luck!