How much would it save in the cost, size and weight of an average flat panel LCD HDTV if you didn't have to include a sound system?

Not very much, because (1) most people want speakers in their TV, and (2) it costs more per unit to build an item with different features. You can already buy TVs without a sound system.  They're called "monitors" (and no, I don't mean that in the *computer* monitor sense, though nowadays they often are essentially the same or at least can be easily used as such -- ironic since that's the same thing we did in the late 1970s).  BTW, the "monitors" cost a lot more than regular TVs.