You probably don't want to use tailscale and other VPN solutions for this, as your friends and any guests would need to install that too to be able to connect.

Do you have a static IP from your ISP?
If so, then as the other commenter said, you only need to set up a port forward to the mumble server running on your network. This is necessary, because that's how you allow certain traffic to reach your server on the internal network. If that Matrix sever is public (internet facing), and federation is working correctly, you probably have a static IP.
If you don't.. well, you would be better off with a domain, then. That's because your IP address will change from time to time, and your friends would always need to correct it. They would get annoyed real quick, I think. But, a domain's purpose is that the computer can look up the current IP address assigned to it, so that should help in that case.

I see lot of interest and ease of installation around matrix and jitsi, however mumble seems not talked about much.

It's because there is widespread interest in a full chat application which includes E2EE, fancy web UI, video conferencing, and integrations with other platforms. Mumble is none of these, it's a rock-solid, old-school, efficient VoIP server with thick clients (desktop and mobile, no working web clients as far as I know). Basic text chat without persistence. Not many changes to the codebase or new features in the last few years. There's nothing very "novel" about it, it just works and is extremely easy to install. This is easily one of my most used services

I am looking for a guide of sort to setup mumble the easy way

• Install mumble-server from your package manager
• Configure /etc/mumble-server.ini and (at least) set a superuser password and a normal user password
• Restart the service
• Optionally setup fail2ban, monitoring, backups, open firewall ports on the server (by default tcp and udp 64738)
• setup port forwarding/NAT on your home router or whatever tunnel solution you're using
• Connect as superuser and create a few channels/ACLs, etc
• Connect using a regular user
• Optionally, reconnect as superuser to add your regular user to the admins group (so that you don't need to reconnect as superuser in the future for basic stuff such as creating channels/moving users/kicking, etc)
• Reconnect as a regular user
• Start inviting people providing them with the address (IP is fine)/port/password of the server

This is my ansible role for mumble installation and management

The Mumble wiki has all the info about other topics.

No need for custom domains, but you need to open and forward the standard Mumble port.