In BigBlueButton, we use the built-in acoustic echo cancellation, so in most cases, you should not hear any echo from remote users. In any event, we always recommend that remote users use a headset with microphone to ensure the best audio in a session.
If a remote user is using a laptop with a built-in microphone, you should not hear an echo. However, if two remote users are using laptops with built-in microphones and neither is using a headset and both are sitting close to each other (close enough for the microphone in one laptop to pickup the audio from the speakers in the other laptop), then you will hear an echo.
Overall, the best solution is ask for all users to use a headset—this will ensure no background noise or echo.