Ask HN: Female hacker/startup looking for copyright advice

Hi all - I'm working on dance choreography site. My vision is in the slogan, "Take a Choreo, Leave a Choreo": users will upload their original choreography, and in return, be able to freely user others' dance choreography - for recitals, dance workouts, small productions, etc. I'm a dance enthusiast and teach a few Zumba classes for fun; I started this site because I wanted something like this for myself.

Here's the link to my rudimentary prototype: http://choreoshare.com/ . There's no choreography on it yet, because I haven't figured out the legal issues...

Specifically - for the videos to really be useful, most of the choreography will need copyrighted audio in the background (Livin' La Vida Loca, for instance), so that people can tell what's happening on particular beats. There's a TON of choreo, with audio, on YouTube; I'm not exactly sure how they do this legally (or if they do).

So, a few questions... one, does anyone know of a way to legally include copyrighted audio in choreography videos, short of paying giant royalties? Is there a chance this falls under "fair use" laws, since the videos are meant for teaching purposes? Finally, does anyone have any experience with or knowledge about altering tracks to make them legal? For instance, the choreo could still be useful if it were synthesized somehow, or missing beats, and had just enough of the original structure to show the user how the choreography is intended to line up with points in the song...

Or, does the burden of providing legal videos lay enough on the user that I could start to promote the site before resolving all the potential legal issues? Is it better to ask forgiveness than permission and go forth boldly, or watch my back?

Many thanks for any advice you can give me. (: - A female hacker