Even though I often advise my students against using music for their multimedia projects sometimes it is necessary. Here is a site that is worthy to explore:http://incompetech.com/m/c/royalty-free/
Make sure you check out the FAQs, mention the creator/company and DONATE to ensure that this music stays free.
Another option is to look for a local band, ask permission to record a track from them that would add to your story and mention the name of the band.
I do not use music in my multimedia stories but I have seen stories on Mediastorm.org that use music.