I don't know of any setups which would support muxing in exactly the way I am thinking, but another interesting use case is if you have multiple incoming audio

I don't know of any setups which would support muxing in exactly the way I am thinking, but another interesting use case is if you have multiple incoming audio streams which you don't want to be mixed by the server -- potentially because they are end-to-end encrypted -- and so a single packet can contain the data from multiple streams. Doing end-to-end encrypted audio calls is finally becoming pretty widespread, and I could see Facebook being in a good position for their products to do custom muxing.