Unable to get Qt5 audio to work using pulseaudio on Rpi 4

Question

I'm on Raspberry pi 4 and the OS is DietPi
I configured ALSA so I could play an audio file successfully through HDMI using aplay [an audio file]
Then launched the PyQt5 app but got PulseAudioService: pa_context_connect() failed. So installed PulsAaudio and enabled its service.

The Service:

[Unit]
Description=Sound Service

[Service]
Type=notify
ExecStart=/usr/bin/pulseaudio --daemonize=no --exit-idle-time=-1 --disallow-exit=true
Restart=always

[Install]
WantedBy=default.target

Now, I don't get PulseAudioService: pa_context_connect() failed anymore, but neither aplay nor in-app sounds work.
I should probably mention that aplay seems playing, because the console's hold (run) time is equal to the length of the audio, but there's no sound.

Answer 1

I could make alsa's aplay work by using aplay [audio file] -D sysdefault:CARD=0 which shows that the default device has changed.
available cards (sinks in PA) can be seen using pacmd list-sinks. For me, cards were:
0- HDMI
*1- 3.5m Jack
in which the default was the wrong one (1).
Make sure hdmi_drive=2 is uncommented in /boot/config.txt if you consider using HDMI.
Default sink can be changed using pacmd set-default-sink [sink index]