So what's the deal with sound?

Little question:

If you built a nice flashwebsite and decide to put in a nice background sound/song/music, where would you get that soundloop?
You probably can’t just put any sound or song you have at home on the internet for the purpose of your website, can you?

How do design studios do this? Do they pay money to some composer to make a tune, or do they pay somebody to use a certain tune?