Transcript:
Schroeder sits smiling at Charlie Brown who is standing over a toy piano plinking out notes. The latter says,"See how easy it is, Schroeder?"
He sits and continues to plink. He says,"The piano is a beautiful instrument if played properly"
He stands up and gestures to Schroeder. He says,"Now let's hear you play, huh, Schroeder?"
He blushes in embarrassment as he sits across from where Schroeder is playing complex classical pieces on the piano.
Trivia:
The first time Schroeder plays a toy piano. The piece played is actually by Rachmaninoff, not by Beethoven
Original comic:
I'd never heard it until I followed your link, so thank you for posting it. I didn't recognize the piece, but used a reverse image search on just that portion of the comic that held the notes. The search came up with no exact matches, but a couple looked promising and I followed those until I found a possible name. Then, on Wikipedia I found this image. It appears to be a match, but I admit I could be wrong.
It really surprised me though that recognizable music was even used at all. He could have just thrown any complicated looking set of notes up and been done. I guess he didn't want to risk the wrath of letters from piano students everywhere
EDIT: Ha! I didn't see that in the original post that Rachmaninoff was mentioned until just now. I thought I was adding new information. I could have saved myself the bother entirely.