Sheldon does in fact sing an octave above Middle C. Either Jim Parsons has perfect pitch, or he had some kind of audio cue to match the pitch.
Sara Rue was on Roseanne (1988) and played Teenage Roseanne on the episode - Halloween IV (1992). Johnny Galecki was in Roseanne by that time but wasn't in that episode.
In the laundry room, Penny asks Leonard what have women said to him when they wanted to slow a relationship down. Leonard says, "I really like you but I wanna see how things go with Mark". In Roseanne (1988), Johnny Galecki (Leonard) played David Healy who had a brother named Mark Healy (Glenn Quinn).
This episode was watched by 10.8 million people with a rating of 3.9 (adults 18-49.)