Quick question, as hopefully this is possible;

I've a Router sat at a remote location with an audio stream on the local LAN of that Router. It has a LAN to LAN VPN connection to a second Router which I am wanting to present this stream from so in essence it NAT translates to a public port on the WAN IP of the remote Router. Is this possible via the Router's Routing function? Obviously I can see and connect to the stream on the LAN of the second, distant, Router (originating and transiting via the VPN) but how would I NAT that to a public port? Do I need to add a static route of it's distant LAN IP in the second, remote, Router?

The reason the audio can't be NAT'd directly from the first Router is because that is double-NAT'd on it's WAN connection and I have no control over the admin side of the Router sat connected to it so it has an outbound aggressive VPN for remote management and other connectivity.

While it may be possible to do what you outlined, the more “conventional” way of achieving something similar would be using a Shoutcast or Icecast server at the second location.

This can be run on a Raspberry Pi and would be on the LAN at the second place taking a stream from the first location. I think this is a relay and sounds like it would be the second case in this document.

https://icecast.org/docs/icecast-trunk/relaying/