Here are links to Apple information regarding enabling the camera and microphone and unmuting a web site.

The first step is to browse to the ViewClix web site: https://viewclix.com/

Then, here is info to enable the Mac's camera & microphone:

And, here is info regarding unmuting the Mac's sound:

BTW:  When it comes to video calling, Apple's browser technology has been several years behind other browsers so we recommend using Chrome or Firefox.