Subwoofer not working in Dell Inspiron 7720

Firstly you will need to install alsa-tools & alsa-tools-gui from either the software center or from terminal.

sudo apt-get install alsa-tools alsa-tools-gui

Open HDAJackRetask

Select the IDT 92HD91BXX codec

Check the “Show unconnected pins” box (the internal speakers do not show as connected)

1. Remap 0x0d (Internal Speaker, which is your Front side) to Internal speaker.
2. Remap 0x0f (“Not connected”, which is the under-display speakers) to “Internal speaker”
3. Remap 0×10 (“Not connected”, which is the subwoofer) to “Internal speaker (LFE)”

Select “Install boot override” to save the settings to apply at boot time. Reboot. When it comes back, you should have full sound from all speakers.