I have a 5.1 receiver that is Dolby DTS certified. Which basically means that as long as the 5.1 audio is encrypted with that algorithm, it's compressed enough to be sent over S/PDIF, which is how I have my PC connected to that receiver.

I noticed that Skyrim seems to support DTS, with very clear 5.1 channel audio being produced, but Fallout 4 does not.

Any info anywhere about this?

I'm having the same problem. Games like Dying Light, Far Cry 3, Borderlands 2 and even Fallout 3 all have fully working 5.1 channel surround. But not Fallout 4. I could of course try connecting a couple of anolog cables to my reciever and see if the problem still exist. Then it has nothing to do with DTS.

I'm using DTS Interactive (real time encoding)

Windows 10 (This was not working in windows 10 from launch but fixed in the recent threshold 2 1511 update)

EDIT: I did some testing and found out that anolog multi channel works. So it seems like a problem with DTS.