They used runtime binary analysis tools to trace raw audio in popular video conferencing applications as the audio traveled from the app to the computer audio driver and then to the network while the app was muted.
They found that all of the apps they tested occasionally gather raw audio data while mute is activated, with one popular app gathering information and delivering data to its server at the same rate regardless of whether the microphone is muted or not.
The Microsoft PowerToys include Video Conference Mute
Quickly mute your microphone (audio) and turn off your camera (video) with a single keystroke while on a conference call, regardless of what application has focus on your computer. Requires Windows 10 version 1903 (build 18362) or later.
BIOS is a software level built upon hardware. How can you know that it is not laying to you, be it on purpose or by a mistake [i.e. a bug]? I always go for the physical kill-switch - thus hate if it is not present
A year ago a had a lengthy conversation with a GIGABYTE about a motherboard of theirs which I was forced to buy [as it was one of the few on he market with a backup BIOS feature built in]. And what do you know- I managed to convince them to update the BIOS, so that the onboard WiFi could be disabled at the post POST level; as going into Windows even once with it operating was making you vulnerable to spying from ECHELON and such
But then again: if Mosad was able ~15 years ago to turn your speakers and motherboard into microphones - what is the point of disabling real microphones, other than hiding your farts from colleagues?