With the removal of the app framework, the Qt mixer demo application needs some form of update, as the agl-service-audiomixer binding that it used is no longer available.  At present it's not clear if the update should be:

1. Refactoring the WirePlumber interfacing code out of agl-service-audiomixer directly into the mixer application.
2. Refactoring agl-service-audiomixer into a new API with some new form of IPC.
3. Other

The disadvantage of (1) is that web and flutter apps would still be in need of something to be able to provide mixer functionality, but (2) while more forward looking wrt that is potentially more work than justifiable for Marlin...

I am assigning this to myself for now, if you have an argument for taking over the investigation, please comment here.