jammendo is great for Music that is (CC) license. There is a bunch of public domain stuff there too iirc. jammendo is also quite cool to get free listening music.
Pro tip: Search for Instrumental, Theme music, or Game music to ensure you dont get crap with "artists" singing. (There is some REAL crap too, so fair warning about the singing part.)
For sounds i recommend getting BFXR:
Pretty fun for making retro game sounds.
You can tweak them further with this:
Just make sure that you export MONO not stereo tracks, there is an option to render stereo into a single track, look in the menu options after you have a sound loaded.
Also, if the game engine supports "Echo" dont make tracks with echos.... You will make a 1 or 2 second sound into a 10-60 second sound which will be a huge file. Just use the initial sound and let the game engine render the echos.
Audacity is AWESOME for sounds that you cant import into C2!! You can make the OGG and AAC files manually if the C2 importer fails. (Which is usually a user failure to remove the stereo tracks, or export at the right frequency.)
Another common mistake is using LAME to export .MP3's.... MP3 is a propitiatory format. If you use it in a commercial game you will owe the license holder lots and lots of money. (I dont think C2 supports MP3 anyway)