Most of these fixes worked properly for me under Kubuntu 20.04 and KDE neon 5.18 on an Acer Chromebook 14 CB3-431! (seems to be good on most Ubuntu-based distros)

All thanks to J. Starnes @ AskUbuntu for the fix.

1. Download the provided asound.state file - link
2. Via Terminal,

pgrep alsa
sudo cp /media/ubuntu/UUID/var/lib/alsa/asound.state /var/lib/alsa/asound.state
sudo alsa force-reload

3. Test to check that your sound is working or not (via Sound Mixer/Firefox etc)/
4. If working, use the following commands to make the changes permanent

alsactl init
sudo alsactl sxitore --file /var/lib/alsa/asound.state
sudo alsa force-reload

Once you have followed the above steps and your sound is working (check after a reboot as well), you have to follow the following steps to unmute your headphones.

1. Open a terminal and type

alsamixer

2. Press F6 (brightness decrease key)
3. Select your sound card (chtrt5650 for me)
4. Press right arrow key once to move to Headphone Channel
5. Press "m" to unmute it and the ESC key to save these changes.

All thanks to optio50 for the fix. Your basic keyboard keys should already be working good other than the exception of the top multimedia keys. To use the keys the same way they work under ChromeOS on your Ubuntu-based distro, follow the below steps.

1. Navigate to /usr/share/X11 folder.
2. Rename the "xkb" folder to "xkb.old"
3. Download the xkb.zip file from here and unzip in the same folder i.e. /usr/share/X11.
4. Reboot your laptop.
5. In your distro's keyboard settings, you should have new settings for Chromebook (Google - Chromebooks).

If for some reason, the boot options/menu show a black screen and you're not able to boot from the USB, try the following fix (big ups to MrChromebox for the fix). These should work on any distro, not just Ubuntu-based.

1. Boot to your installed linux distro
2. In Terminal, type the following command:

sudo efibootmgr -O

(capital O and not a zero)

3. Reboot and you should be able to see the boot menu/options now.

P.S. if you're able to see the EFI shell, the following command might also work (haven't tested):

setvar BootOrder =

and then type

exit

Reboot and you should be able to see the boot menu/options now.