In meeting, my audio and video have specific problems

During a meeting, others can hear me, and I can hear others, but I can't hear myself.  And in the meeting, when others talk, their video moves to center screen and becomes enlarged.  When I talk, it does not.  It stays as a thumbnail in the upper portion of the screen along with the non-talkers.  Before the meeting, when I tested my audio, the two tests worked (the ringtone test and the recorded playback test).  I'm using the mic built into the webcam, and using headphones to listen to my audio.  But I cannot hear myself speak in real time through my headphones, as I would expect.