Ian, good point...I forgot about the tempo and transpose options available on keyboards with audio backing capability.

It's my understanding, you have options with these audio backing tracks.

1. Full original recording to learn a song.

2. Minus one instrument so you can be the fill in guy

3. No vocal,(right hand) so one can sing or play along.

All of that sounds similar using recorded styles and midi's, only this is in audio format. To me it sounds like fun, and at the same time you get to be a part of the original recording.

Lip Syncing to someone else voice? Who does that professionally? If it's part of your act and the audience knows that, then maybe, but if you're trying to fool the audience that your Frank Sinatra, they will know...nobody's that's good.