If you're the only one speaking, but you hear that echo sound, sometimes you can look at the microphones in the participant list and see which one is bouncing a sound level. If the sound isn't loud enough that might not suffice.
Then I would recommend if feasible to go around one person at a time and mute them. That's assuming that they were initially unmute. Once you mute the offender the echo will stop.
Suggest that they use earbuds, headphones, possibly turn down their speakers, and for sure have them confirm that Original Sound is not ON.
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