Garageband for iPad in the end. I had just used that for my video class, so I decided that would work for this too.
The only way that I think you could include voice acting or singing is if the voice actors or singers were completely anonymous. However, someone will know, since sending audio files isn’t anonymous.
I actually don’t know what chords I used since the piano portion is a Garageband loop. However, I can knock it down an octave - all I need to do is export the loop by itself into Audacity, knock it down an octave and reassemble the track. The track itself isn’t all loops - I actually played the drum part with an actual drum and recorded it.
As for Mediafire, it’s just http://www.mediafire.com I don’t think it has a chat feature, but I think it has a file sharing function which I haven’t used. Obviously the @moderators would have to approve if it were to be more widely used. You can, however, put any type of file on there, excel sheets, Word Docs, etc...so it could get out of control.