If you're looking for a software solution, you might do this:
Or this:
This plugin will allow you to automate various tasks using "Macros". Macros consist of a list of conditions under which a list of actions will be performed. Examples and guides can be found in the wiki. Feel free to contribute! If you run...
obsproject.com
Note that the latest Beta of Adv. SS contains an "else" section, so those two macros can be combined. My version is probably the last official release that doesn't do that, but you can still make it work as shown here.
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If you'd rather have a hardware solution (I'm a bit of a software guy myself, but there's some value in avoiding the Rube Goldberg machine that most software is under the hood), then you might get some ideas from here:
ESP Project Pages - Microphone Mute Circuit.
sound-au.com
ESP Audio Projects - Reverb Mute System - mute the input, with a configurable soft-mute at the output
sound-au.com
ESP Audio Projects - AC signal muting and/or switching using bipolar transistors - strange but true.
sound-au.com
ESP Audio Projects - Project 104 Preamp or crossover muting circuit
sound-au.com
Main site:
DIY Audio from ESP - Audio articles, projects to build and general information about hi-fi and audio in general.
sound-au.com
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All of that said, anything reactionary is not going to catch everything, which makes the ones that do get through, that much more noticeable. Even an otherwise uncalled-for dropout can call attention to it, where it might have gone unnoticed otherwise.