I think in keeping with the classical music angle, I'd pick the Elvira Madigan concerto by Mozart for Quentin's grammophone. It's very sad yet romantic, the way his theme was. It would fit Quentin and Beth's relationship I think.
But if we're going for rock/pop music, "Take Me In Your Arms" by the Doobie Brothers would fit perhaps for Beth.
EG, the music nerd (been a music nerd since age 6 and still going strong LOL!)