The Guitarist Amplification

Summary
An argument between Leonard and Penny proves "inconvenient" to Sheldon. We learn that Sheldon's parents argued a lot when he was growing up, and he doesn't like to hear people arguing at all. Penny had invited a former lover (male guitarist) to stay at her place without talking to Leonard beforehand, which Leonard was quite upset about. Everyone else was arguing, too: Raj and Howard, Raj and his parents, and Howard and his mother. Sheldon even went to the Cheesecake Factory by himself to try to talk Penny into apologizing to Leonard so he could get on with his life. At the end of the show, Penny's friend is seen sleeping on Sheldon's couch as Leonard and Penny are in bed together in her apartment.

Title reference: Penny's friend's occupation as a guitarist.