Because BigBlueButton audio and video travel through different channels there is a known glitch that may impact BigBlueButton audio and visual synchronisation. If you are experiencing problems with sychnronisation you may need to try changing your webcam settings.
BigBlueButton webcam settings are
automatically set to Low definition. It
is recommended that when you join your
BigBlueButton session you click on the
webcam button at the bottom of the screen,
and choose High, or High definition to
improve your session.