Here is a thought

Soapvox

Want some of my Kool-aid?
I am not sure if they have it yet but you might be able to find a way to create an interface in real basic where you could use an iMac as a touch screen client. OS X and Server are both Ideal soplutions I think for what your friend wants to do because each person could sign up on a webpage and using PHP and MySQL, you could create a very easy interface for them to upload their music, then when they get to the bar they could only browse the mp3 folder, also the stability is amazing.
 
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