I found great background music sounds here:
http://incompetech.com/m/c/royalty-free/index.html?keywords=015&Search=Search
But you will need to process them because most are pure and uncompressed.
Audacity is a great open source audio editor. Export the sounds in mp3 format and compress them so the files become reasonable in size to download and still sound ok.
You may also want to mix down the stereo track to mono. It cuts down on size a lot and for 3D sound effects it won't matter so much.
I hope this helps.