The make your own cards are really nice for this. We bought a bunch of them and you can add any mp3s you want onto them. We even print stickers to put on the front.
Adding to the plethora of good ideas here: My wife bought these hanging tabs to stick onto the cards[1], and then strings a keycable[2] through them so my son has groups of them together. Yoto makes folding binders for them as well, but the keycable method seems to be a bit easier for our 5yo to handle.