Here’s a couple I’ve seen around, tho I haven’t dived too deep:

• OpenGameArt (Usually kind of a mixed bag on quality, but the site has good concept and intentions)
• https://musopen.org/ (public domain, classical stuff, also a bunch of sheet music if you want to do things the hard way)
• https://degitx.com/ (an individual that produces rock/metal/chiptune kinda stuff and puts it out under CC licenses)
• https://gravitysound.studio/ (Had it bookmarked, but it seems the free page is gone or at least moved. Looks pricey, and terms don’t seem clear)