Most likely:
- the audio codec is configured wrong
- your soundcard has a very weak reference clock.
Try e.g. if it works in Windows Media Player or using Wave Output (do not forget to change the codec as well then).
Even the cheapest realtek onboard audio does work okay for S/PDIF using MPA (choose 16 bit) or ffdshow.
Only some Asus boards which use e.g. ADI (Soundmax) solutions are known to cause issues.
__________________  |