This one requires a bit of an explanation. In Lord of the Rings Online you can play as a "ministrel" whose function is to play "ballads" using various instruments, primarily in order to produce beneficial effects on your party's morale and fighting ability.

One of the instruments you can choose to play is the horn. Though for some reason (I imagine it's a joke) it's held like a saxophone and sounds like one too. This, in combination with the fact that the "ballads" you play are just second-long bursts of random notes makes it sound like you are accompanying beatnik "poetry" in the middle of a pitched battle.

And that is hilarious. Even more hilarious than the cowbell, but less pleasant-sounding actually.