Description
Problem: There are environments that require multiple "departments" to coordinate with each other, but they don't always have a hierarchical relationship with each other that lends itself to abusing channel linking and ACL. And besides, abusing channel linking and ACL is complicated and pushes the limits of what these features are intended to do in the first place.
As an example, let's look at a game I play called Artemis, a game where people work together as the bridge crew of a space ship, and if you have enough people you can crew multiple ships and have them all work together. Each ship wants to have their own channel for coordinating the operation of their own ship, but the captains of all the ships will also want a channel to coordinate fleet actions, the science officers will want a channel to coordinate how they want to split up scanning duties so that nobody wastes time scanning things that other people are already working on, the fighter pilots will want a channel to coordinate their attacks, and none of these people want to have to listen to each other. There's no way to do this in, really, any voip software out there short of extremely expensive matrix intercom solutions aimed at television broadcasters.
I can imagine other large scale games like EvE, ARMA, etc. run into similar issues.
Solution: Let the user join multiple channels at once. From a usability perspective I would create the notion of "channel slots," where you would right click on a channel and select "join in slot 2", "join in slot 3," etc, and bind push-to-talk and other functions to act on the additional slots. The normal channel switching method of double-clicking on channels would continue to work as it does now so that users can continue to use Mumble the same way they always have, but channels joined that way would be considered "slot 1."