Youtube’s Audio Library offers music and sound effects for your videos.

From what I can tell, you are free to use the music and sound effects in multimedia productions:

By downloading music from this library, you agree that you will not:

  • Make available, distribute or perform the music files from this library separately from videos and other content into which you have incorporated these music files (e.g., standalone distribution of these files is not permitted).
  • Use music files from this library in an illegal manner or in connection with any illegal content.

The music and sound effects can be downloaded for your use, with the caveat that some of the music is Creative Commons licenced and requires attribution of the song. These songs will have a  icon in the listing.