The following tests were done on 3 different displays - A monitor with built-in speakers, a capture card and a TV. All of them exhibited the same behavior, and worked perfectly fine when the devices used for testing were directly connected to them.
The first upscaler I got a hold of was a VP30 with the De-Interlacer Card: I tested it out of the box and managed to get a picture but heard no sound. Everything I threw at it came out mute. So I updated it to the latest firmware, but HDMI still produced no audio. All of this is regardless of cables/settings/etc. - e.g. if I have my Saturn plugged in (Stereo RCA+SVideo) and the VP30 set to AUDIO IN: Stereo + AUDIO OUT: HDMI, I'll only get video and hear no sound at all. However, even if I do HDMI passthrough it doesn't want to listen, where plugging in a MegaSG with the appropriate I/O settings enabled in the upscaler it STILL produces no HDMI audio yet the optical out produces sound. Go figure.

The next DVDO product that found it's way to my home was an Edge Green: I immediately fired it up hoping this would be the end of my HDMI audio nightmare only to get immediately smacked in the face with yet another "Video but no Audio" situation.

Both devices were factory reset after their respective firmware updates to ensure it wasn't a configuration problem, and I want to reiterate that connecting the Saturn/MegaSG directly to these 3 displays all produce video and audio just fine, albeit with terrible internal upscaling (which was the point of buying the DVDO). Tried the 1919 trick, the audio offset trick, letting them warm up/cool down/unplugging them, etc. All to no avail.
Am I missing something here?
