It could be that this particular TV was originally meant for industrial or educational use and is set up to use an external sound system. Can you see jacks on a panel on the back, side, etc. marked "audio", "L/R" or similar? If so, you can try hooking up the audio outputs from one set of these jacks to your stereo system, if you have one. It seems to me that you have already found the video in/out jacks, as you mention RGB video inputs in your post; the audio in/out jacks will probably be in the same area, if this set has them.
BTW, how did you get your plasma TV in your house if it weighs so much?
Where will you put it when you resolve the audio issues? If you plan to put it in your living room, I hope you have one the size of the state of Texas, since you're going to need it (and sturdy floors) if the TV is as big as and weighs as much as you say.
I cannot believe you can't find
any information on this TV. There has to be a model number somewhere, either on the cabinet or inside it, since some company made the unit. Have you tried doing a Google search for "Sole 55 inch plasma television" on the Internet? I would think there would be at least
some info on this TV online. There is a chance, however, your set may be an "orphan", made by a little-known company for which little or no info is available; I think this one might fall squarely into that category.
The other thing I'd do is to look inside the set and note whether or not there are any recognizable markings or brand names on major components or anywhere on the chassis. As I said, some electronics firm, somewhere in the world, made this TV, but finding out just who the manufacturer was will obviously be a challenge.
Good luck. Your set is basically working if you say you can get a good picture. If worse comes to worst with the sound issues, I'd just connect the set to an external audio amplifier and speakers and use it that way. Many people with TVs as big or bigger (!) than yours quite often connect the set to an external stereo system (or specially-designed A/V receiver) and have had good results. CBS television advertises many of their prime-time programs as being presented not only in HD, but 5.1-channel surround sound as well. This is overkill, IMHO, for most programs on television these days (who wants to listen to the CBS Evening News, for example, in 5.1-channel surround sound?), but the signal is there to be sent to a compatible stereo system, if desired.