Subwoofer stopped working on Dell Inspiron 15-7567

I'd suggest diving into the 'Sound' settings in the Control Panel first. Once there, right-click on your playback device and open 'Properties'. Under the 'Enhancements' tab, ensure that 'Bass Boost' and 'Loudness Equalization' are activated. If they aren't, it's worth giving them a tick and applying the changes.

Next, you might want to peek into the Realtek Audio Console. Navigate to the speaker configuration settings there and double-check that the subwoofer hasn't been inadvertently disabled. On the driver front, while you're in Device Manager looking at sound devices, try right-clicking on Realtek Audio, select 'Properties' and then move to the 'Driver' tab. There might be an option to 'Roll Back Driver', which can revert to a previous driver version that possibly worked better for you.