This would require a bit of web programming wizardry.
The screen would show a graphic representation of a mixing board with the title of the song to be crowd recorded. There would be sequenced guide tracks that would be replaced by people's uploaded performances of that track. If you played cowbell,
you'd play your own cowbell track and upload it. Other cowbell players would upload their takes on the song and participants would vote on the best track. The track with the most votes gets the top slot and at the end of the competition, will be the featured track in that song for that instrument.
To draw people into the competition, a famous musician might be featured on one of the tracks. You might therefore be competing to be the bagpipe player on a Paul McCartney song playing alongside Sir Paul himself. (Paul actually has a great song featuring the unfairly maligned bagpipes called Mull Of Kyntire)
Back to the graphic of the mixing board, you'd see a track marked "Guitar" and thumbnail pictures of the top ten guitar players lined up vertically. To review their submission, you simply click on their picture to highlight it. You then vote on it either with a thumbs up or down or ranking it's quality from 1 to 5 stars.
The song would then be mixed and put up for sale, perhaps some of the proceeds being given to charity, this being done to help lure big name stars to participate. Obviously the big star's track would be set and not up for competition.
Backing up Bruce Springsteen for a new rendition of one of his hits might be Joe Johnson from California on guitar, Bjorg Bjorgonson from Holland on drums, I.C. Wiener from Fartlandia on bass etc.
But seriously ladies and germs, this could be a neat way for people all around the world to come together and make music which although sounding a little corny would actually be kind of cool.
Live versions would also be performed over the internets with the players who won the competition actually playing in real time alongside their favorite stars from the comfort of their homes in their respective countries.
Possible names: "Planetary Rock Fest 9,000" Alternately just "Planetary Rock Fest" or "Planet Rock". "3rd Rockfest From The Sun", "Earth Rock 9,000", "Earth Rockfest", "Earth Rockfest 9,000", "Earth-Fest 9,000". Ummm. Ok I'm done.