What part of "other duties as assigned" do you not agree with, that's already in your contract and stipend agreement? You're already getting paid for your playoff games.

It's in fact illegal/against UIL rules for coaches to make extra money when/if they make the playoffs. Why would it then be turned around and OK if the band director got extra pay for their playoff games? That would give you a vested interest in changing the outcome of the athletic contest in the limited ways we're allowed to do so.