Audio feedback - or echoing - happens when a session participant's output audio device (mic) picks up his input audio (loudspeaker). That results in other participants hearing an “echo” of everything they say. When you hear the echo, you can not remove it yourself, as someone else in the session causes it. It can be fixed only by the participant that is causing it. You can spot the session participant causing the echo by monitoring the nametags of the participants. When a participant's nametag is constantly turning green when you are speaking, he is not, and the echo happens, usually, it means that his microphone is picking up your voice and your loudspeakers are playing it to you. You need to ask him to fix the issue to get rid of the echo. Fixing the issue means simply preventing the used microphone to pick up the session audio.
In the desktop mode, the common case is where a computer's built-in mic and loudspeakers are being used for audio. The best and easiest way to avoid the issue is to use a good quality pair of headphones with a microphone. Please note that also some Bluetooth headphones w. a mic are known to cause issues with output audio.
With VR devices lowering the device's loudspeaker level helps.