Just in case, I tried to install Qt6 natively, but that installed version 6.2.4. However, from what I can see, your app. comes with its own set of libraries and the Help » About Qt menu shows 6.4.3.
[Error ] [consumer sdl2_audio] Failed to initialize SDL: Could not connect to PulseAudio
It looks like this is your problem. You can check if it is installed. It would be surprising if it is not installed because PulseAudio is installed by default on Ubuntu and many applications depend on it.
Looking around, if pulseaudio works, then the following should have worked:
$ pactl info
Connection failure: Connection refused
But as we can see, I would get an error instead. Probably meaning that either pulseaudio is not running or the connection is down, somehow. (something crashed?)
What I ended up doing since the currently running service was hung is look for the process with ps to find the PID and kill it: