If you can hear others, then your speakers are working fine. You must be refering to your microphone. In every meeting you either start or participate, your michrophone is muted, you need to manually unmute it by clicking on it.
On this article you will find further information on how to play around with your Audio Settings. If you are using an external microphone, then try with the integrated one.
In case the problem still persist, we recommend that you un-install and re-install the Blizz application.