This is the puppet-scale Grand Piano from Team Base. It appears to be a completely custom made piano (although the keys appear to be from a toy piano). It features a top, legs and music stand made out of clear plexi-glass. There's also a computer print-out of a photograph of the inside of a grand piano on the inside. The keys actually work, and hit a series of metal rods that make a 'tink' type sound (but each one is a different note). It was originally blue, but they re-painted it dark brown (and that's the color it's seen in the movie). Some blue is showing through scratches though.

The top can be closed just like a real grand piano.


Here's a screen shot where you can see the piano on the left

And these are three publicity images:
The first one shows the complete Team Base set (with puppets).
The second is basically the same shot, but also include film makers Matt Stone and Trey Parker sitting on the set.
And the third one is another behind-the-scenes shot showing the piano in it's original blue color.
(Click on any of the three images to view a larger version of it.)