So when you hear echo... look at the talking indicators of the other users, and see which ones pulsate when "you" talk.. your voice is coming back thru their mics.
AEC echo cancellation works on all the users clients and tries to cancel your voice coming out of other speakers so it doesn’t enter back into the users mic and into the conference.
Easiest fix is just mute the offender and echo disappears.
So why does this happen? The echo cancellation has a buffer to try and handle jitter and do its job.... if some of your users are on lossy network connections, and the jitter is > 160ms, the echo canceller loses it sync, and has to retrain. The jitter was larger then the browser buffer. (not selectable yet in browsers)
Also if a phone joins in by telephone call, and it is on speakphone, the same thing can happen if it does not have AEC built into the phone device.
You often will notice when a user joins, you hear echo for just a second then AEC kicks in and does its job...
Regards,
Stephen