The webinar has started, but there is no sound

If the webinar has started and you can see Kaya, this usually happens because the Integrated VOIP  has not been switched on when entering WebEx. To do so, you need to go into Menu:  

Communicate -> Integrated VoIP -> Join Conference   

If the option Join conference is not available, try disconnecting and re-connecting in the Menu as follows:

Communicate -> Integrated VoIP -> Leave Conference then Communicate -> Integrated VoIP -> Join Conference

If it still does not work, re-starting your computer and re-joining the webinar can help.