Freesound is a collaborative database of audio snippets, samples, recordings, bleeps, etc. that have been released under Creative Commons licenses so that you can use them for free. You could use this to find sound effects for things in your game (like someone eating an apple or a space laser firing) or even some atmospheric background music.

As always, you should check the Creative Commons license on each thing you want to use matches your needs (as some licenses don’t allow for commercial use) but there’s guidance in the FAQ on the Freesound website.