One I don't have every song known to man....
Then you don't play it -- simple.
Let me think back through the Alzheimers for a moment -- oh yes, back when I used vinyl, 8-tracks and cassettes. Somebody asked for a song, and I didn't have it.
OMG, what did I do... Asked them for their next choice (and probably didn't play that either)...
Entertainers are not immediate gratification specialists. Tell them to buy an iPod if they want to listen to that crap...