Details
P1: Critical Description
Look: https://doc.qt.io/qt-5.10/qtexttospeech.html#volume-prop at volume property is defined as double, but notifier signal is declared as
void volumeChanged(int volume)
Simple way to workaround is connect this signal to some slot like: void function(int) and in this slot get current volume value from QTextToSpeech object (you have got access to it in this case).