I discovered that the MediaPlayer didn’t contain the possibility to adjust the playback rate/speed which is covered by the libVLC functions:

libvlc_media_player_set_rate
libvlc_media_player_get_rate

The needed functions in MediaPlayer.cpp would be:

void VlcMediaPlayer::setPlaybackRate(float rate)
{
libvlc_media_player_set_rate(_vlcMediaPlayer, rate);

VlcError::showErrmsg();
}

float VlcMediaPlayer::playbackRate()
{
if (!_vlcMediaPlayer)
return -1;

return libvlc_media_player_get_rate(_vlcMediaPlayer);
}

and accordingly in MediaPlayer.h:

in public: float playbackRate();
in public slots: void setPlaybackRate(float rate);

Is it possible to implement this functionality?