As I mentioned, I suspect that this is an active choice, as just displaying the icon whenever a media context exists seems much easier than inspecting the audio stream for non-zero volume media.

I can only assume that there are legitimate reasons for this as well, e.g. websites preparing/maintaining audio context for lower latency when they intermittently play audio etc.

Not just notified but blocked by default. We already have similar bocks on autoplaying video (no audio until you interact) and access to microphones and cameras ("allow access.." popups).

Every time an article like this comes out, people wonder about aliexpress instead of why their browser allows this in the first place.