Music and other sounds coming from speakers even when computer is off

Hi All,

I am running the iMac Retina 5K, 27-inch, Late 2015 with the Harman Kardon soundsticks bubble speakers. About 5 months ago I would get random music sporadically playing for a few seconds here and there and then go away. I ran the normal scans with Malware Bytes and it seemed to go away. However in the last month the sounds have become less sporadic and are now playing full time on the speakers. The noises sound like someone is playing a game in the background but the quality of the audio is fantastic so I dont think its picking up a neighbour. The interesting/scary thing is it still plays when the Mac is switched off. You also cannot change the audio volume through the mac so I think (this might sound weird) that I have a virus in the speakers themselves??? Is that even possible and if so how the **** do I get rid of it?

Any help would be greatly appreciated.

Matt