I have a Dell inspiron 7720 notebook. There is also small subwoofer on it but worked well only left and right side of speakers on ubuntu 18.04.2. Then I used these steps:
sudo nano /etc/pulse/daemon.conf
I uncommmented and changed the values of these lines:
enable-lfe-remixing = yes default-sample-channels = 3 default-channel-map = front-left,front-right,lfe
Then I edited the following file: sudo nano /etc/pulse/default.pa
Added the following line to the end:
load-module module-combine channels=3 channel_map=front-left,front-right,lfe
After restart I saw in sound settings new option for subwoofer and when I pushed Test sound button, nothing happened. Then I used hdajackretask and overwrote the pin ID 0x10 with Internal Speaker (LFE). Install boot override and restarted notebook. I went back to sound settings and when I tested subwoofer ubuntu gave me back whitenoise sound. In alsamixer I can not see option for subwoofer only through gui sound settings and that was strange for me. But suddenly after few days my subwoofer started working so i was happy, but next day when I turned my notebook on an tried to use volume down button, sound is still at max velocity. In Gui sound setting appeared new soundcard and I must everytime change through settings, sound, choose a device for output for change a volume. When i plug earphones I need to do it again. Any ideas how to setup to work correctly?