The Convention Conundrum - S7-E14
Sheldon Cooper: [sneaking up to a house] I don't understand what we're doing.
James Earl Jones: Shhhh!
Sheldon Cooper: Whose house is this?
James Earl Jones: Carrie Fisher. And she's a little crazy, so get ready to run.
[He rings the doorbell, and they run for it.]
Carrie Fisher: [storms out with a baseball bat] It's not funny anymore, James!
James Earl Jones: Then why am I laughing?
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.