Answer from Gigabyte support:-
"Dear Sir,
X79S-UP5-wifi doesn’t support Dolby and DTS."
This is somewhat disappointing considering the ALC898 is a newer chipset than the ALC889 I previously used.
According to the GA-X79-UD5 specs, it also uses the ALC898 chipset which does support Dolby, so this can hardly be a hardware limitation as Gigabyte support suggests.
In my searching however I can see there are attempts to work around this with modded drivers "Realtek/Soundmax Modds + SB X-fi MB"
If I get it working I will post my results.