The following guide has been provided by our user Pier Luigi Covarelli. The steps have been tested with the XBian latest (2005-10-17).
Step1: remove the default sound module into /etc/modules.xbian by commenting its line entry
sudo vi /etc/modules.xbian
Comment the line
Step 2: edit /boot/config.txt file by adding the proper overlay
sudo vi /boot/config.txt
Make the following changes:
Step 3: Reboot.
Now you can select the DIGI into the Kodi Audio Output panel.
If you use a HiFiBerry DAC/DAC+ or Amp, you need to adapt the dtoverlay line accordingly.