I found a picture of a medieval organ at this site.
I saw this picture a few years back in Audsley's The Art of Organ Making, along with a few other pictures of medieval organs. This was my favourite, due to the monk on the left wagging a finger at the men pumping the bellows.
As you can see from the picture, what constituted an organ back in that era is rather different from what we consider to be an organ today. It is much smaller, with fewer pipes, yet it requires two men to play it. It also needs four men to pump the bellows. The pumping could cause the air pressure to fluctuate as the organ is played, and it is possible this is the reason why the monks are trying to correct the bellow pumpers. The nature of the playing mechanism is unseen and unknown, although by this period organs had stopped using the sliders of the original Greek and Roman Hydraulus and used large keys which the players pounded with their fists. The early organ players were hence known as 'organ beaters'.
At any rate, your mission, should accept it, is to come up with the dialogue of the scene, for either the monk on the left or the right. Here's the best I could come up with.
"One more time, and I revoke your baptism."
Monk on the right: "Stop whacking him in the head! He's already deformed and a foot shorter than you."
Sigh. "Three legged, two headed midgets just don't pump like they used to."