Realtek Audio Drivers for Ubuntu

The best thing to do would be to remove alsa and reinstall it. The same thing happened to me when I switched motherboards, and this fixed it.

1. Remove the ALSA packages.

    sudo apt-get --purge remove linux-sound-base alsa-base alsa-utils
   

2. Reinstall the same packages.

    sudo apt-get install linux-sound-base alsa-base alsa-utils
   

   Packages gdm and ubuntu-desktop are also removed in this process if you are using Gnome. They need to be reinstalled:

    sudo apt-get install gdm ubuntu-desktop
    # or 
    sudo apt-get install gdm3 ubuntu-desktop
   

   If you are using Xubuntu this will also happen to you.

    sudo apt-get install gdm xubuntu-desktop
   

3. Reboot.

    sudo reboot
   

I had the same problem (internal mic not working) with my Lenovo Ideapad 320, which runs Ubuntu 18.04.

I installed GNOME ALSA Mixer from the Ubuntu Softwares Panel and followed another topic instruction: put the "Capture" full on the right and "Internal" empty on the left, as the following image:

It worked well for me.