I think Yoda got used to our surround sound. We have a big screen as well. Well, for regular TV and cable, there's not much surround sound (only TV speakers). But when we watch DVDs, it's usually in full surround sound. Yoda doesn't really get alarmed by the rear sounds now.
What he does do is bark like mad whenever a dog or HORSE (heh, we watched the Kentucky Derby and Yoda yapped the whole race) is on the screen. He wants to protect his territory and alert me there is danger (in his mind). I appreciate it, and then tell Yoda a stern NO and "Down, Yoda"... He also barks when he hears doorbells on TV and commercials. He thinks someone is coming to his house and wants to tell us about it. The funniest is when Yoda's tired and napping, then sees it on TV, he'll pop his head up and give out a few barks then plop his head back down again, job well done.
A few times we've caught Yoda sitting on the bed watching TV intently. It is amazing to catch him like this, as he's intensely focused on his Animal Planet program. Must not have been a horse or dog on, as he didn't bark at all. :P