Using power lines
To transmit data is nothing new. Back at the turn of the century high-frequency signalling was used to turn street lights on and off in many cities. (Turn of the 19th/20th century, I mean).
GE has marketed telephone extenders which send the telephone line signal over the household 120V AC line for many years. They work, too.
Various firms offer both audio, modem and network connections over the AC Mains.
As long as the appropriate filters are used, the whole thing is perfectly safe and reliable. After all, here you have this enormous (by communication standards), thick cable, firmly connected and running everywhere in the house already. Why not use it? Our "M-Net" here in Munich uses this for telephone, TV, DSL and, yes, 230V AC. Here's a link to a review from 2002, about the time the network technology had become stable and cheap:
http://www.pcworld.com/article/id,85003-page,1/article.html
And here a link to an interesting review on applying this American technology to the European situation:
I have read that you must not skip from one 'leg' to the other without signal loss (anal retentives please spare me the PC language lesson, sigh) but have seen it work just fine with network connectors here in Germany, so go figure...
I am curious, tho'. Is there anyway to safely open one of those speakers and take a look inside? At that point in time, Magnavox was still a serious audio components producer with genuinely unique ideas and technology.