There isn’t, actually. But I can tell you that it’s fewer and fewer as time goes on. It seems like a relic of the old Livejournal/MySpace days. Most viewers nowadays are either playing their own music or they don’t have speakers.
The other thing with automatic background music on site is that if someone is surfing from work, people around them know immediately when they are surfing the web instead of doing their work (unless the keep the volume down or muted).