If the sil viseme is set to -none- on the current avatar, then the user's microphone input is echoed back in their speakers. (I haven't tested on other visemes).
Note that this only occurs if the avatar is changed after initial load - it appears that if your first avatar on vrchat startup has an invalid viseme, this does not occur.
Tested on live and open beta (838).
Viseme設定が-none-など、不正のシェープキーに設定されている場合、マイク入力がスピーカーに反響するようです。