Xrayleader
these sites are good sources of textures and PBR materials, as well as skies and some high quality models, but they are mostly for 3D. all of the assets are cc0 (public domain) so you can use them for whatever you want.
https://www.sharetextures.com/
https://polyhaven.com/
https://ambientcg.com
https://3dtextures.me
the site www.textures.com (formerly cgtextures) was one of the most popular for game developers, specially AAA, but has become obsolete unless you buy materials, as only plain textures are free. It also has a license with many conditions, one is that you can't bundle the textures in a GPL game, or use them in certain ways. Godot is MIT so they should be compatible as long as you don't convert your project to GPL. but at the end of the day it's up to them to allow this or not.
for sounds I usually go to freesounds and look for public domain sounds or ones with a compatible license. or just record them myself on my phone.
for music you can try jamendo, the songs have licenses, and for a commercial game you can contact the artists and agree on a license, or at least that's what it used to say, I haven't been there in a while. These days I try to make music with MuseScore 2, either my own or use notes from public domain music. you can also find public domain music (usually old, like folk or classic) around the internet. just make sure the file is public domain, a song from 300 years ago can still have copyright if it was played by someone from this era.
OpenGameArt.org is good, you can find some of my old assets (just a castle wall and one other texture). assets there tend to be poor quality or disjointed (or used to), but it's all free.
