This can occur due to an Anti Virus or security restriction on your PC which does not recognize VidyoWeb and blocks the media traffic.
The issue is that video isn’t displayed for the external end user via the web.
I can see them and hear them without any issues logged into VidyoDesktop.
After disabling the Anti Virus the call works as expected.
Vidyo recommends to allow media traffic to and from the VidyoWeb Plugin if possible, if this is not possible then you would have to use Vidyo Desktop as a work around.