edit permissions, run on admin, double check ports, reinstall, make mumble support ticket in that order
permissions-right-click , properties and i think the security tab ports are in your IP address. Just type it in your browser. it should be somewhere on the mumble website or whatever what ports have to be open
I don't see why your ISP would be blocking Mumble though. It's a standard VOIP application. If you can use Skype and so on, then you should be able to connect. It sounds like there's an application on the PC or the Router maybe that's actively blocking the port you would connect to Mumble on.
Well, if you think Comcast is blocking you, you can of course proxy, but Mumble works fine for me, and I use Crapcast. You could try opening up the port from your router (port forwarding).