Why can't I click "share sound" when sharing a video in some webinars?

[training_mental]

I understand that I'm supposed to click Share, then click the box for "share sound." To share a video, I will then click the tab for Advanced, and then Video File button, then Share button.

But in a webinar today, after the host gave me the permission to share, I was able to click Share, but the "share sound" button could not be clicked. It was not checked. It was not grayed out. It was empty, but could not be clicked. I was able to continue to Advanced/Video File/ Share, but of course the participants could not hear my sound.

On the same machine, with the same device settings, I later started a webinar with just myself as the host, and I was able to Share and then "share sound."

Is there a setting for the meeting itself that needs to be changed?