Soundation is a online music mixer. You can mash your own music or you can use the music loops provided.
Check out the Learn section of the site to find out how this web application works. I'm not a musician or have experience with music software so it did look a little overwhelming.
I can see what to do but I think an additional video tutorial demonstration would be helpful.
To get you started you can use the 400 musical loops for free to build your mixing skills. Later, you can purchase professional loops that will give you more options.
Here is the good part. After you create your music you can use it however you like, in the video, commercially or whatever. My understanding is that the service will create an mp3 file and provide you with a link to download the file for your production.
This might be a good option for those of use that want personalized music for videos.