You can use music that is royalty-free. These can be found on websites and in music asset packs on Itchio.
You can use music from games you've previously made, because you have permission from yourself to do so. And because the jam allows you to use assets you've made before the start of the jam.
With using soundtracks from other video games you didn't make, I can't answer yes to that question, because of possible legal issues with doing that. If you don't own the copyright, the original owner can request that you remove it from your game.
