The goal should be that all Qt Multimedia APIs work cross-platform with no surprises. We need to identify what the set of APIs are that users need to cover the 90% case, and which are supported on all backends. Some features may be supported on all platforms, but are largely unused and buggy, such as the Qt Audio Engine. Others may be required for many users but are only possible to support on some platform. We should find out what is what and categorize them.
This is a good starting point: https://wiki.qt.io/Qt_5.11_Multimedia_Backends