Using traditional headsets is a great idea. You should also IMO look into a software-only solution, i.e. a music player that streams its output to other smartp

Using traditional headsets is a great idea. You should also IMO look into a software-only solution, i.e. a music player that streams its output to other smartphones via bluetooth or wifi. Because if your idea takes off, this is IMO what will kill a hardware product. And you might eventually talk to accessory makers to build such capability into cases.