The Big Bran Hypothesis - S1-E2
Audio problem: When Penny is talking to Raj on the stairs to the apartments, during some shots we see her from behind. In one shot, we hear her say something like "And I'm freaking out", but you can tell by the way her jaw moves that she was actually saying something different. (00:14:50)
The Hamburger Postulate - S1-E5
Audio problem: When Leonard and Leslie continue to practise together after the other two musicians have left (Leslie now being the only violinist), vibrato from a violin can be heard - from observing Leslie's left hand she is not applying it.
Audio problem: When Sheldon is knocking on Leonard's door at the start, the sound is out of sync with the action.
The Barbarian Sublimation - S2-E3
Audio problem: When Penny walks out after Sheldon helps her with her game, we see Leonard start the music but the speaker doesn't start vibrating for over a second after the music has started. It should have started at the same time. (00:07:40)
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.