Sound stopped working after upgrading to Linux 5.4 (Intel HD Audio)

Here is how I was able to resolve this:

1. sudo vim /etc/default/grub
2. Find GRUB_CMDLINE_LINUX_DEFAULT and add snd_hda_intel.dmic_detect=0 to the end of it. (ex: GRUB_CMDLINE_LINUX_DEFAULT="loglevel=3 snd_hda_intel.dmic_detect=0")
3. sudo grub-mkconfig -o /boot/grub/grub.cfg
4. Reboot the system.

The option snd_hda_intel.dmic_detect=0 is deprecated on newer kernels, you may use snd-intel-dspcfg.dsp_driver=1 instead. But be aware, it's slightly different meaning.