This solution is applicable if the system has SPDIF (optical audio) being used and the amplifier or other device connected to the SPDIF port complains about lack of signal or goes too easily to power save mode and such behavior isn't wanted. The problem is caused by the default powersave setting of the snd_hda_intel kernel module which will rather quickly turn the SPDIF port off if no audio is played. If this module isn't being used in the system then this solution isn't likely to work.
The fix is to disable the automatic powersaving mode of the snd_hda_intel kernel module:
echo "options snd-hda-intel power_save=0" >/etc/modprobe.d/snd-hda-intel.conf
Note that “snd-hda-intel” is used in the command while the module is named snd_hda_intel for example in the `lsmod` output.
Remove and reload the module or reboot the system to have the new options enabled.