Re: Speaker sysem
There is little doubt that there was something in the trailer originally to supply sound to those speakers. If you can't locate any possible place where a radio or something could have been, try pulling out the speakers to see where the wires go from them. That should at least point you in the right direction. I can assure you that plugging a radio into an outlet will not supply anything to those speakers. There have been devices which sent an audio signal through the power lines, but they require special equipment both to add the sound to the power line and to separate it back out. And they only supply a very poor quality of sound. Friends of ours have an intercom which does exactly that, sending the voice over the power lines. But you can hardly understand the voice if you are not close to it and you would not want to play music that way.
I would look for signs of something having been mounted in or under one of your cabinets.