It appears at this time that Apple’s default for Do-Not-Disturb is “Always on” until you turn it off, and when you switch it to a different expiration, the next time you set it, it goes back to “Always on”. When the moon icon appears, you can click on it to set your preferred Do-Not-Disturb expiration. during your meeting, but will allow your Mac to still play your meeting audio through your speakers or whatever your desired sound output is. Turning Do-Not-Disturb on will silence all the beeps, etc. The little moon icon is identical to the Do-Not-Disturb (DND) icon on your iPhone, one of the many ways Apple is making the overall Mac and iOS experience more seamless. Instead, just option-click on the day and time in the upper right hand corner of your display, and when you do this you’ll see a little moon icon appear in the menu bar. Most people will instinctively try to turn off sound, but that just results in not being able to hear the meeting, which is no bueno.