Someone emailed Realtek support and got a reply?!
I'm truly amazed.
OK, so in my Acer/Realtek HD Audio Manager I just have two tabs - Speakers and Microphone (a third, Stereo Mix, appears if I enable that option in Vista's Sound > Recording Devices tab). The Speakers tab has a slider for the main volume only; the Microphone tab has a slider for the recording volume only.
If I go to Vista's sound options, the Playback tab has the speakers as the only playback device. If I go into Properties > Levels for the speakers, the only volume sliders I have are Realtek HD Audio Output and PC Beep. The Recording tab has two devices: microphone and stereo mix. Neither of those have anything that would allow me to listen to the mic input.
The main Windows mixer (accessible from the System Tray) just has sliders for Speakers and Windows Sounds.
Like you say, it doesn't look good! I'll take a look at the VAC program, thanks for the tip. FWIW this is what the previously mentioned section of my registry looks like:
Can you see anything in there that looks remotely promising?