I believe that nobody made or tried full working S/PDIF system with most several formats for gaming in pure DOS. Therefore I decided to explore this problem as exhaustively as it's possible. In 2014 I finished this work and made a sound system with SB Pro, SB 16, Yamaha GM/XG, EMU8k standards and all of them in S/PDIF, working together.
I used in my system:
1) Terratec EWS64 XL - I must to say that this card doesn't make real digital signal, it converts its own mixer through 18-bit ADC to S/PDIF signal. BUT: sound quality is real perfect! And due to this it is possible to connect to EWS64 line/cd in other analog devices and get a joint signal to S/PDIF out channel. AND: on Terratec EWS64 it is possible to switch digital in source. It can be (a) - card mixer ADC, (b) - coax in, (c) - optical in.
2) AV310 CMI8330, digital "SB 16". It has S/PDIF IN and OUT. I connected EWS64 spdif out to AV310 spdif in and got joint signal from EWS64 and AV310. EWS64 is making sound in SB Pro format, AV310 in SB 16 format.
3) AWE64 Gold - it can give us only MIDI sound through spdif. I connected it to EWS64 S/PDIF IN COAX. Also it is possible to use Goldfinch CT1920 card.
4) NEC XR385 with S/PDIF mod by jwt27. Perfect Yamaha digital sound! It is connected to EWS64 S/PDIF IN OPTICAL.
Also I found:
The SB Live! card can play spdif sound in pure DOS. But not all revisions and not all DOS driver versions. It is necessary to try different combinations of card and driver revision. EMM386 is required.
I have my own review of assembled and full working S/PDIF sound system in pure DOS. It is in Russian.