As reported by @cqexbesd, BEL is not working, even though QApplication::beep() is being invoked. Googling for that turns out that this doesn't seem to always work and other people's fix was to simply not use it but other APIs, e.g. QMediaPlayer from QtMultimedia.

This seems as simple as:

QMediaPlayer player; 

// ...

player.setMedia(QUrl::fromLocalFile("/usr/share/sounds/freedesktop/stereo/bell.oga"));
player.setVolume(50); 

// ...

player.play();

But while being at it, why not making it (per profile) configurable, such that users can tweak what's about to be played when BEL is being processed.

NB: In case of BEL spam, a new sound must not be played if an old one is still playing.