We probably do have the same backup system. I tested the sound by turning the volume all the way up, didn't get any sound out of the speaker at all, not even a hissing sound (although I was exercising the generator at the time, maybe I just couldn't hear it). The old monitor doesn't appear to have a speaker like the new one (it's in the back).
I replaced the camera last year (Apr 2013), model 313.C; I didn't see anything that made me think it might have a microphone, although according to the specs it should have one. As I said earlier, it may have to do with 8-wire vs. 13-wire cabling. I really don't know, just speculating.
I'm going back out to the rig later this week, I'll look at the manual and play around with the settings some more. If I learn anything, I'll post back.
Since it appears the old monitor wasn't working very well from the get-go ("pre-owned", we purchased in 2012), I'm very happy just to have a usable monitor, and since I didn't have sound to start with, I'm not really missing it. (We do have walkie-talkies when we need them.) But it it's there, I'll find it, and let you know.
Doneworking: I really looked into replacing the whole system with a wireless setup. But I'd already invested in the new camera just a year ago, and I never saw any reviews for wireless systems that made me want to discard the new and start over. I have no issues with wireless, but it would have taken a 6 stars out of 5 rating to get me to switch
. Obviously, I didn't find one like that.
If I decide to go wireless in the future, I would seriously consider a Zone Defense system. They have one with a 7-in monitor. And their systems are aimed primarily at truckers, secondarily at RVers. I would think they would not have interference problems with all the other "stuff" in an RV (electronics, inverters, converters, satellite antennas, what-not), plus would be able to handle all those potholes out there trying to eat our rigs. But that's just more speculating on my part.
And based on my recent research, today I would be very suspicious of a $79 wireless camera! If something sounds too good to be true...
But be sure to post when you do decide on your next system. I'd be far more likely to trust info on the PC Forum before most reviews elsewhere, even Amazon!