I think the Proton ad is good, but misleading. TONNES of apps *specifically* get microphone access despite being apps that wouldn't seem to need to require it. (Photo editing app for example).
So your mic is, 100% being used to harvest you. Let's assume it. In most normal cases.
Yes, behaviour profiling doesn't require much access, and is powerful. And is not widely understood nor cared about, but mic access makes it easier for Random Company XYZ to collect and sell data. Very specific data.