In order to get some sort of audio output, I changed back to the only HDMI monitor having speakers in my possession. After the removal of pulseaudio (apt remove pulseaudio), the monitor is perfectly fine to use for sound playback, however, the audio from those tiny speakers is pretty poor. I guess, it is what it is.
Hopefully the Raspberry Pi Foundation will solve the Bluetooth issues with the RPi400 as soon as possible.
