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The issue is that the `qt6-qtmultimedia` package in version 6.4.1 was compiled without support for any of the supported audio backends. Here's a slightly shorter version of what I wrote over on the MKVToolNix issue tracker (issue https://gitlab.com/mbunkus/mkvtoolnix/-/issues/3448):

The error message appears when the Qt multimedia library wasn't compiled with any of the supported backends (gstreamer, PulseAudio, ALSA, WMF on Windows…). This should obviously not be the case on a Linux distribution.
I just gave this a try with a brand new VM:

* downloaded the Fedora 37 Desktop ISO
* booted it, installed it to the hard disk with default options
* rebooted into the installation, set up my user
* logged in
* installed my repository with sudo rpm -Uhv https://mkvtoolnix.download/fedora/bunkus-org-repo-2-4.noarch.rpm as described here
* installed MKVToolNix v72 with "sudo dnf install mkvtoolnix-gui" (at this point a couple of extra packages were installed, among them qt6-qtmultimedia)
* started mkvtoolnix-gui, and it came up just fine

At this point the installed versions of the Qt6 libraries are:

adwaita-qt6-1.4.2-1.fc37.x86_64
libadwaita-qt6-1.4.2-1.fc37.x86_64
qgnomeplatform-qt6-0.9.0-2.fc37.x86_64
qt6-qtbase-6.3.1-4.fc37.x86_64
qt6-qtbase-common-6.3.1-4.fc37.noarch
qt6-qtbase-gui-6.3.1-4.fc37.x86_64
qt6-qtdeclarative-6.3.1-2.fc37.x86_64
qt6-qtmultimedia-6.3.1-2.fc37.x86_64
qt6-qtsvg-6.3.1-2.fc37.x86_64
qt6-qtwayland-6.3.1-4.fc37.x86_64


So we have no crash with 6.3.1.
However, Qt 6.4.1 is available in the "updates" repo, which is enabled by default. So I updated everything with sudo dnf upgrade. Afterwards the library versions were at:

qt6-qtbase-common-6.4.1-1.fc37.noarch
qt6-qtbase-gui-6.4.1-1.fc37.x86_64
qt6-qtdeclarative-6.4.1-1.fc37.x86_64
qt6-qtwayland-6.4.1-1.fc37.x86_64
qt6-qtbase-6.4.1-1.fc37.x86_64
qt6-qtsvg-6.4.1-1.fc37.x86_64
qt6-qtmultimedia-6.4.1-1.fc37.x86_64


Now I do get a crash with 6.4.1.

Downgrading to 6.3.1 with "sudo dnf install $( grep '^qt' libs-before.txt | sed -Ee 's/.[^.]+$//' )" immediately fixes the issue: the GUI starts again just fine. Also:

[mosu@fedora ~]$ ldd /usr/lib64/libQt6Multimedia.so.6.3.1|grep strea
        libgstreamer-1.0.so.0 => /lib64/libgstreamer-1.0.so.0 (0x00007f54fbadb000)
[mosu@fedora ~]$ sudo dnf upgrade
…content cut…
[mosu@fedora ~]$ ldd /usr/lib64/libQt6Multimedia.so.6.4.1 |grep stream
[mosu@fedora ~]$

Version 6.4.1 is indeed not compiled against gstreamer anymore.

This affects both the package provided by Fedora 37 itself (v71.1.0) & the one from my own repository (72.0.0).

Similar problem has been detected:

Just run the app

reporter:       libreport-2.17.4
backtrace_rating: 4
cgroup:         0::/user.slice/user-1000.slice/user/app.slice/app-org.kde.konsole-c7f91e50297547bbaa32ebb2bf65cecb.scope
cmdline:        mkvtoolnix-gui
crash_function: qAbort
executable:     /usr/bin/mkvtoolnix-gui
journald_cursor: s=8891d0f8535148a8b155067c370b4c80;i=1b38;b=ae5341260f784d238f186df9c76330e9;m=73c2f71d5;t=5ef2ce16e4bd4;x=394a2af3a1ab9cc7
kernel:         6.0.11-300.fc37.x86_64
package:        mkvtoolnix-gui-71.1.0-1.fc37
reason:         mkvtoolnix-gui killed by SIGABRT
rootdir:        /
runlevel:       N 5
type:           CCpp
uid:            1000

(In reply to Moritz Bunkus from comment #12)
> The issue is that the `qt6-qtmultimedia` package in version 6.4.1 was
> compiled without support for any of the supported audio backends. Here's a
> slightly shorter version of what I wrote over on the MKVToolNix issue
> tracker (issue https://gitlab.com/mbunkus/mkvtoolnix/-/issues/3448):

this gave me the idea:

$ dnf downgrade qt6\*
...
Downgraded:
  qgnomeplatform-qt6-0.9.0-2.fc37.x86_64           qt6-qt5compat-6.3.1-3.fc37.x86_64               qt6-qtbase-6.3.1-4.fc37.x86_64                 
  qt6-qtbase-common-6.3.1-4.fc37.noarch            qt6-qtbase-gui-6.3.1-4.fc37.x86_64              qt6-qtdeclarative-6.3.1-2.fc37.x86_64          
  qt6-qtmultimedia-6.3.1-2.fc37.x86_64             qt6-qtshadertools-6.3.1-2.fc37.x86_64           qt6-qtsvg-6.3.1-2.fc37.x86_64                  
  qt6-qtwayland-6.3.1-4.fc37.x86_64               

and mkvtoolnix-gui works again.

and
$ ldd /usr/lib64/libQt6Multimedia.so.6.* |grep stream
        libgstreamer-1.0.so.0 => /lib64/libgstreamer-1.0.so.0 (0x00007fe3af164000)

Will be resolved once the update from 2150231 hits the "update" repo.