Explore the possibility to use VLC when building Elisa for desktop targets. As far as I can say, its should be easier to provide a good out of the box experience especially for flatpak and Windows platforms.
Digikam switched to QtAV once and they said it's better for cross-platform. It also has a QML interface and a similar API to Qt Multimedia. I think you should check that first before VLC.
I have also been given the advice to use libVLC.
In all cases, I just want to play music not video and with a dependency not needing to install audio codec on Windows or with an easier way to bundle all the codecs like for flatpak.
Is QtAV a good idea for audio only needs and with an easy way to get all audio format supported ? I honestly do not know but I will have a look.
I forgot for a second that you don't need video support. Well I'm not entirely sure if QtAV supports many audio formats, but it depends on ffmpeg which I believe support many. I have very little knowledge here and just wanted to add some info about the existence of QtAV :)