Sound effects:
- https://github.com/saniv/free-game-sfx
- http://freesound.org/
- https://annotator.freesound.org/ (=freesound.org, but more searchable)
- http://opengameart.org/
- https://gamesounds.xyz/
Music:
Disclaimer: Some of these websites (those with the exclamation mark icon) have some files that are not libre, among all the libre files. But you can filter them out in the search function, to filter out the bad licenses. Use the filter. So make sure to understand the website. I included thsese websites because they still offer a huge wealth of libre files, it's just that not 100% of their files are libre.
I hope I could help you!