I suppose one needs to see the character in context within the game. To me the "mood" is very evident. It's quite clear that for a brief moment he is very angry. I was thinking that a little smoke or something from the increased friction when the organ plays quickly could be very effective, but you are correct, it would draw more attention to the character which may not be your intention.
Well that wouldn't work out to well. Part of the humor in the game is to just see the subtle snap in his expression and speed up. Having his music box change would be like having him flat out say "I AM CURRENTLY IN A BAD MOOD." It'd defeat the purpose of him having quick little bursts of anger.
That music still gets stuck in my head over a decade after playing that game too much.
BadaDEE - BadaDEEE - Deee badada beebadeee
When he gets angry I'd love to see the organ itself transform somehow.
Yay Ocarina of time windmill guy