http://creativecommons.org/legalmusicforvideos
everything on this planet is going to have some restriction. If you want to do it right which god I want to poke my eyes out on this forum right now use music which grants permission for reuse.
if you want to monetize your video use music in this list that allows for commercial reuse
if you dont want to monetize pretty much everything there is fair game
You can also learn to make your own background music using garaband or something.
for sounds for your videos if you want to add a sound there are huge repositories for this
the two I use are
www.soundbible.com and
www.freesound.org
again check the licenses before using them but for the most part you can use them.
when you use this stuff take notes. At the end of the video put the information in the credits. Provide a link back to those requiring you give them credit in your youtube page as well.