The /flash boot partition is read-only by default, so we need to remount it in read-write mode:

• mount -o remount,rw /flash
  Use the nano text editor to modify the file. Save changes with ctrl+o and exit using ctrl+x:
• nano /flash/config.txt

add the following:
dtoverlay=pisound
dtparam=audio=on

After editing set the /flash partition back to read-only mode:

• mount -o remount,ro /flash
  And reboot for the changes in config.txt to be applied:
• reboot After reboot you can change in Kodi to headphone.