I know a lot of people want to use Foundry's built in support for A/V. I also know a lot of people struggle to make it work. I'm not going to go into the reasons why it's perhaps complex to use, that would be a fairly technical post, but what I'm going to suggest is that people consider using the Foundry Jitsi Module instead.
Getting it working on a site which supports TLS (all Worldmill Mills do), is really simple.
Access your mill and, if you haven't already, install the Jitsi Module. It's a standard module and you'll find it when you search through the list of Add On Modules. The full name is "Jitsi WebRTC Client".
In your foundry world activate the Jitsi module. Nothing more required than clicking a check box.
As gamemaster, use "Configure Settings" to enable Audio/Visual support. Also check to make sure that "Configure Players", then "Configure Permissions", has the checkboxes ticked so that players may broadcast audio and video.
Each player must enable Audio/Visual support and make choice between "Always Enabled"/"Push To Talk"
I had some trouble with my keyboard and push to talk. I don't know why - hardware. Anyway, my top tip is try a few different keys until you find one that works. I use my "F1" key, since I don't need it for anything else.