I don't recall any music/sound freebies, can you share the file path of what you're referring to?
Golly, you can use the ones in the dlc/baseassets folder, and anything that generates with a fresh project. The point is use content that everyone has. If you went to steam and purchased a copy of MV (or any other rpgmaker engine) by itself (unbundled), the files included with that purchase are usable.