When Randy Marsh yells out, "No!" to Stan and smashes his head into a glass case, it is in fact Patrick Stewart's voice taken from Star Trek: First Contact (1996) in which Stewart as Captain Picard yells out, "No!" smashes a glass case in with a phaser rifle.
When Stan sits on his parent's bed, you can see a Visitor in the painting behind them above the bed. The Visitor is hiding behind one of the beams of the bridge in the painting.
The boy playing the piano at the auditions is Schroeder, the Beethoven aficionado from Peanuts.
"Ghetto Avenue Boys" and "Avenue Ghetto Street Boys" parody Backstreet Boys, and "F*ngerbang' parodies **NSYNC'.