Hi Alan,
Theoretically, you could use solid wire, or even metal coat hangers. They just aren't practical, or made for the application.
Even though, copper is copper. Someone went through much trouble to make all the different styles, and types of wire; and for good reason.
Unless you get Romex NM cable for free, it isn't worth using. Even then, it's too hard to work with, in comparison to speaker cable. (as was mentioned)
I suppose you could run it under the house, and into the wall, then to a speaker jack wall plate.
Then go with speaker cable from there.
As for 12 or 10-AWG; it really depends on how long the run is.