alright so this might seem kind of silly but i think it still might be fun. you could sing into the mic of the phone and it would play your voice along with the music through the headphones. this might limit some of those people in public places who love to sing along, not really knowing how bad they really are! haha
I think this sounds awesome, but let me make a few changes. It doesn't have to play your voice back, surely you can hear your own voice. But it needs to play the music and have the works synced on screen. A lot of work but I would definitely buy this in the App Store.