I use Arch Linux, Plasma 6.5.2, Qt 6.10.0, KDE Frameworks 6.19.0, Linux kernel 6.17.7, Wayland.

I notice that ffmpeg qt6-multimedia backend doesn't work for the sound, I get the message "no audio device detected" in the console when I run software (Gwenview, Dragon for example) that uses this ffmpeg qt6 multimedia backend, and same issue when I compile Qt6 source code that uses Qt6 multimedia backend,

the output on the console when ffmpeg backend is used, note the message "no audio device detected" (complete output is provided with this ticket as attached file) :

 
qt.multimedia.ffmpeg: Using Qt multimedia with FFmpeg version n8.0 GPL version 3 or later 

(...)

No audio device detected

I tried to set environment variable "QT_MEDIA_BACKEND" to "ffmpeg", but it doesn't fix the problem,

but if I try gstreamer multimedia backend then I have sound :

export QT_MEDIA_BACKEND=gstreamer

I don't understand why ffmpeg Qt6 multimedia backend fails to detect my sound-card (Asus Xonar SE PCIe sound card),
I use Pulseaudio as sound server.
VLC player works (I have sound), MPV, Mplayer work also for the sound, the problem is for softwares that use Qt6 multimedia backend (when ffmpeg backend is used).

I have installed all the required Arch Linux packages for ffmpeg Qt6 multimedia backend  :

qt6-multimedia-ffmpeg 6.10.0-2
libvlc 3.0.21-31
vlc-plugin-ffmpeg 3.0.21-31
vlc-plugins-all 3.0.21-31
ffmpeg 2:8.0-3