If you are hearing an echo, it's likely because you have the live webinar opened in two or more places.
If you are the presenter, make sure you aren’t listening to the session from the audience’s point of view aka the front-end of your session. Your phone or VOIP system could be picking up the audio from your computer. It’s best practice to only have the Presenter’s Screen open during your session.
Echoes can also be caused by the physical space you are presenting from.