Experimente os seguintes passos:
1.
echo 'options snd-hda-intel model=asus-mode4' | sudo tee -a /etc/modprobe.d/alsa-base.conf
2.
sudo gedit /usr/share/pulseaudio/alsa-mixer/profile-sets/extra-hdmi.conf
e adicione as seguintes linhas ao extra-hdmi.conf
[Mapping analog-surround-21]
device-strings = surround40:%f
channel-map = front-left,front-right,lfe,lfe
paths-output = analog-output analog-output-speaker
priority = 7
direction = out
3.
sudo gedit /etc/pulse/daemon.conf
e adicione a seguinte linha ao daemon.conf
enable-lfe-remixing = yes