Used to describe the terrible noises that you must endure in a public restroom when you walk in and someone (or multiple persons) are in the process of taking a dump. Or, it can simply be an allusion to using the public restroom.
"I had to take a leak the other day, but the bathroom choir was in full session, so I got out of that restroom ASAP."
"I'll be back in a second. I have a practice session with the bathroom choir."