Echo where?
- There's the classic one where you have a Desktop source (Settings -> Audio) or an Audio Output capture (part of a scene) that looks at the same device that the Monitor is being sent to. That creates an audio loop.
- There's another possibility that your mic is picking up your speakers through the air. Most meeting systems have an "echo canceller", that knows what's going out to the speaker and detects that coming back from the mic, in order to take it back out of the mic. But these echo cancellers are very sensitive to the acoustic arrangement. If you use a different mic or different speakers, they might not work at all, and you have that echo being sent back to the remote people as if you actually said it.
I had the latter problem with one of my rigs, that has a mic in the middle and the speakers in a TV at one end. I really wanted both for their own reasons, which meant that the echo canceller was useless. So I made my own crude one by ducking the mic under the speakers, HARD!
I use a DAW now, but when I did it in OBS, I needed two compressor filters back to back, both set *very* aggressively, and side-chained to the meeting return, to squash the mic enough that the remote people were okay with it. (and of course, make sure that the meeting gets its audio from those compressors, and not directly from the mic)
The side-effect of doing it that way, is that the remote people can talk over the local one(s), but the local people can't talk over the remote one(s). That's been okay for me, but you'll have to consider if it's okay for you.
