Tldr; There needs to be a master audio level display or at least some sort of master indication whether a stream is getting an audio signal or not.

Or at the very least, audio sources should be muted by default in new scenes.

We have an Intro screen scene that just displays our logo and some background movement with a message that we will begin soon. We started the live stream and then muted the mic on the audio inputs and then, for good measure, muted the physical mic. We then proceeded to chat and get things ready for the presentation, etc.

Little did I know that OBS includes -all- audio sources on every scene, by default, unmuted.

And though I had muted our regular mic, the webcam's built-in mic was on and transmitting. We didn't see the green audio level animation or even the listing for that input either, because it was at the bottom of the list of audio input sources where you have to scroll down within that box to see it.

Luckily, we didn't say anything too embarrassing, but it was embarrassing nonetheless.