Here's some steps you might want to double check. Networking, port forwarding, and the like can be somewhat tedious at times.
- Double check that the port is open. Try using this open port checker just to confirm. I've seen weird situations where it's not possible to connect to a server running on the same computer using the external IP. Get another computer involved.
- If the previous step fails and the instance of the Mumble server is binding correctly, it's probably an issue with port forwarding or a firewall. If this is the case, the first thing I would do is disable Avast and remove the Windows firewall (or any other security measures). Again, this is not safe permanent option, but good for narrowing down the issue.
Edit: Since the port is open according to the open port checker, try connecting from another computer. Does it work?