I've been doing a lot of research on music libraries for us home musicians.
What is a music library you ask ? In summary it's a catalog of music that is made available to movie, TV and internet companies that may require music from 1 second to full song. All that music in those commercials, or that song playing in the jukebox in the background of a scene form CSI or the music on a news show, all of this could have come from a music library.
One way to get your music into a library is through a service like Taxi and Taxi has a produced a lot of videos on this available at youtube.
Many of these videos talk about how they may only use a few seconds of a track but that this can generate a nice income. One example was a snippet used that offered only $3 per airing, but that ran on a cable channel constantly through the day for weeks generating $3k for the songwriter in a single quarter.
Another, free, service is Pump Audio I went through their signup process and it's a lot of work but who knows if it could be worthwhile.
I really recommend watching the youtube series of videos called "Earning Your Living with Film & TV Placements" from the Taxi set. They are very interesting and talk a lot about how, with some attention to quality and some effort, you rally can make money from your hobby.