One of the more common questions in flash game development is where to get free sounds or music.
Probably the best free sound archive I know of is Soundsnap. They have tons of sounds effects, sound loops, and music samples. And they’re all free to use in your games!
You can find almost any sound there, and if you can’t find exactly what you’re looking for, you can use Audacity to the modify the sounds until you get what you want. The program is open source and ultra easy to use. I get particularly good results from mixing together a couple of similar sounds to get a richer sound effect. But be careful not to mix too many or else it’ll sound disorganized.
As for a free music site, I don’t know of an all-encompassing site like Soundsnap but I do like opsound. They have a good selection of nice music there. The songs are covered under the Creative Commons License so it’s best to to ask for permission before using it in a commercial game. Most artists will be happy to be featured in a wide release game so it shouldn’t be a problem to ask.