The Sony HDR-AS30V has extremely high audio gain, which is precisely what you need for the mic on an action camera. It sends that audio, along with the video, through its HDMI output, so when you drive a display from the camera in enclosed space, the audio is REALLY LOUD and causes severe feedback. For obscure reasons, given the staggering cost of the venue’s AV system, there’s no way to mute the audio channel of the video input when you’re also using a mic attached to someone giving a presentation.
The obvious solution, a shorted jumper (formerly an earbud plug) in the external mic jack, looked like this:
Contrary to what I expected, the camera doesn’t disable the internal mic with the jumper in place. The amp probably uses an analog multiplexer, rather than a mechanical switch, and even an off-channel isolation of, say, 76 dB (from the MAX4544 spec, for example) isn’t enough to completely mute the mic. You could, given sufficient motivation, measure the actual isolation, but the surviving audio isn’t subtle at all.
The not-obvious solution turned out to be putting the camera into either single or interval photo mode, rather than the movie mode I use for bike rides. It seems that when the video format doesn’t require audio, the camera either disables the audio inputs or (more likely) just doesn’t include audio data in the HDMI output.
Which produces exactly what I want: a video output with no accompanying audio.