You could try wording it like this, "So no matter how many times he's told to play "Free Bird" those requests will be in vain, just like a real live musician"
This almost gives K.K. some humanization, and kind of makes him sound like an easy going person.
I'm not sure how it would be worded, but I mean for it to sound like this, "just like a real live musician ;-)".
Edit: It also explains that it's a joke without going into it that much by comparing him to a real live musician.
I just want to clarify, that by "humanization", I mean that it compares his skills to that of a real live musician. So it's kind of doing two things at once.
@Polo: It's the background song that he does not know.
K.K. has around 50 or so songs that he can play, and if you request something random like "Duel of the Fates" or "Battle of the Heroes", then he'll just play an "untitled" song. I put untitled in quotations, because every "untitled" song that plays (3), becomes a new song that you can request in the next Animal Crossing game. (Yes, I put a Star Wars reference in there xD).