Cambodian Rock Band, Lauren Yee's new off-Broadway play that uses music to talk about the Khmer Rouge genocide, is about a lot more than history and trauma. Written by an Asian-American playwright with a mission to bring joy to stories of survival, the play is directed by Chay Yew and sports an entirely Asian-American cast, led by Courtney Reed and Joe Ngo, whose parents were labor camp prisoners under the K…