That's usually due to one thing: Unmuted microphones can create an echo. It is generally recommended in online meetings to use headphones. We also highly recommend muting your microphone if you are not actively participating in an exchange, as extraneous (background) noise can reduce ROOM's optimal experience significantly.
ROOM also has a built-in noise cancelling feature, make sure it's turned on for you by clicking the three dots in the bottom right during a meeting, and then clicking "Device Settings". In the popup that comes up, make sure "Noise suppression" is on, as seen below.