Shawn Montano, a news video editor who blogs here, has a wonderful, hands-on post about how he uses music in his stories. Among his themes: Music can really help a story, but it’s easy to abuse. I would add that it’s often better to create your own music, or find someone to produce what you need, than it is to stumble through online directories of canned music.
And don’t forget: NEVER steal music! That is, don’t use music that you don’t have the rights to use. The results can be costly.