Can't hear sound? You likely have more than one sound card and ALSA chose the wrong one at boot. To correct this, you could create a .asoundrc file in your home directory specifying the default card. Unfortunately, Firefox may not recognize this file so you would continue to have no sound in Firefox. To remedy this, you will need to specify via modprobe in which order to load sound drivers so that your preferred card is selected first by ALSA.

First, run the following command to see which order ALSA assigned your cards:

cat /proc/asound/cards

In my case I got:

0 [HDMI ]: HDA-Intel - HDA Intel HDMI
HDA Intel HDMI at 0xe0510000 irq 61
1 [PCH ]: HDA-Intel - HDA Intel PCH
HDA Intel PCH at 0xe0514000 irq 59

Now, tell the kernel to load PCH as the default:

sudo bash -c 'echo "options snd_hda_intel index=1,0" > /etc/modprobe.d/alsa.conf'

You can verify the driver name to use above by running

lsmod | grep snd

Reboot the computer and you should have sound!