FreeSFX.co.uk is great for high quality sound effects and background music. Check their license page for the legalese; the gist seems to be that you can use them so long as 1) the site is in your game credits, 2) there are visual elements, and 3) sound isn't your core game mechanic.
(Their search engine is kind of garbage, so if you find a track you like, you might want to check its creator's profile. They've probably published other versions of it that the engine isn't finding due to a lack of tags / etc.)
I recommend downloading Audacity (its free) in case you need to edit/balance the lengths/volume/fade/loop of any sfx or music you're using. (It's obvious when you throw stock audio in without any [much-needed] editing.)
And if your project needs photography / icons / fonts, The Stocks archives pretty much all major free stock sites in one place. Be sure to check their individual licenses.