Update: Since I wrote this post, I made a website for choralplayer here: http://choralplayer.appspot.com/
Over the last couple of days I built a flash application I’m calling Choral Player. It’s basically a music player that can stream multiple sources of music simultaneously.
It’s for choirs, for people to learn their parts. You record each part (say sop, alto, tenor, bass) separately (they have to be in time!), then use this to allow people to play the separate parts simultaneously. They can change the levels of parts during playback, so they can just listen to their own parts, or theirs loud with others quiet, or maybe drop out their part and sing with the other parts. Whatever is useful.
If you can record separate parts into separate mp3s and get them online somewhere, you can use this app. Just have a look at the “choral player” page, look at the page source (“View Page Source”), and you’ll see the list of mp3s, the list of voice names, and the title; you can customise these to your own purposes.
Have a listen and a play here: Choral Player.