I get my audio from Open Game Art. Highly recommended. All of it is under free culture licenses, and people are diligent about making sure of it. And much comes from actual games, like canceled projects, so it's pretty usable.
Viewing post in Gathering and implementing audio assets
Awesome, I will check it out tomorrow, thanks. You mention "free culture", though. I intend to ultimately sell my game when it's finished (with another free demo, just like the current pre-release demo is free). I hope that the site allows for this, even if it means I have to pay a reasonable license fee.
Yes, free culture licenses allow commercial use by definition. You might have to release any modified version of those assets you've used under the same license, and/or allow people to extract them from your game for reuse even if you didn't modify them. And if that's too much to ask, you can always contact the original authors for a commercial license. Hope this helps!