I don't have it in hand at the moment, but the 2820 does have a setting for automatic volume control relative to speed...I would assume the Zumo does as well. I can't even remember how I got there on the screen, but the option is there.
Equons...I'm not real clear on what you are trying to accomplish here. I have an additional AUX input on the back of my HK radio (thanks to hd-dude) with the 3.5mm stereo output jack from my 2820 fed into that jack instead of the one on the front of the unit. Voice commands can come through my J&M headsets or through the speakers, depending on Switch position. With HK in AUX mode, voice commands from the Garmin, and music from it's MP3 player feed through the AUX input. Voice commands override the MP3 when needed. I'm sure I'm missing something in what you're talking about doing here, so enlighten me further, please.
What there is a market for, IMO, is something to integrate, with STEREO output, the Bluetooth capability that comes with the new Garmin units (both listening and talking), a good quality set of helmet speakers (don't have to be Bose, but at least as good as J&M's....better would be better), a good microphone, and make it all work without being tethered to the bike with a cord to the helmet. For both rider and passenger.
Make something that will do that, and I'd pay good money for it, particularly if the headsets were high quality, at least approaching the level of home type units. I very rarely listen to music or Garmin voice commands through the bike's speakers any longer. But, I wear a Nolan full face 98% of the time, as does my passenger.