Finding a website that provides free music or sound effects for commercial use (under public domain or Creative Commons Zero license) is very, very hard, and there are only a handful of such websites right now.
Most websites with free audio content use some sort of Creative Commons licensing, but very rarely it is CC0 license.
If you need free photos for commercial use, check this out: 7 websites to get free images for commercial use. If you need free vectors and illustrations for commercial use, check this out: 7 websites to get free vectors and illustrations for commercial use. If you need free videos for commercial use, check this out: 5 websites to get free videos for commercial use.
CC0 is the only Creative Commons license compatible with GPL license. So, here are 5 websites with high-quality music or audio files, all under Public Domain or CC0 license, and free for commercial use, with no attribution required.
Use comments to let me know if you have some other CC0 music and sound websites to recommend.