If I start a session and then change the volume or mute setting then that works fine as long as I'm connected. However if I disconnect and reconnect again then those settings are lost. This is probably not what the user wants.
The issue gets extra complicated if I reconnect from a different machine. Is the expected behaviour to get that machine's last volume setting? Or the session's? I'm leaning towards the machine as this considers the volume a property of the application, not the session. That is similar to how a video player keeps the volume even for different files.
There is also the issue of the system remembering the volume for us. PulseAudio does this on Linux¹, and I think Windows does as well. So this might be a matter of interacting properly with the local sound server.
¹ Doesn't seem to work properly using a casual test. Works if I adjust the client entry in e.g. pavucontrol locally. But it doesn't work if I change the volume from inside the session.
Not to be confused with bug 2045, which is about providing such a service for applications running in the session.