The most accessible Microsoft documentation relating to writing a driver (MS-DOS or otherwise) for the Microsoft Windows Sound System AD-based hardware is MSDN #94396 Windows NT DDK 4.0 Workstation US (01/1997) in the folder
/ddk/src/mmedia/sndsys/. It includes a complete reference driver for AD1848-based cards, accessing the ports directly, as expected. It's intended as an example for writing multimedia drivers under Windows NT, but it does show the process of acquiring the card and beginning output.
Microsoft published several tiers of documentation and support for WSS in 1993, including at least the Windows Sound System Driver Development Kit (DDK) (for $450) and the MS-DOS Developers Kit.
Dr. Dobbs carried an advert for them, and Microsoft list it on their history, but I haven't been able to find either of them. It would be ideal if the MS-DOS Kit contained a commented MS-DOS-ready driver with a readme indicating step-by-step minimal examples for startup and configuration, but we won't know until it's found.
The screenshot in Geo...'s answer is of the MSDN Library Archive 1997 disc - a big hypertext WinHelp-style interface with a lot of programming information, and some release notes, bulletins (such as this one marking the release of the DDK) and odds-and-sods. Sometimes it's comprehensive, sometimes it's not. The screenshot itself shows the Windows Sound System Version 2.0 DDK Release Notes that would accompany the DDK, but unfortunately the DDK isn't present on the disc itself. Part of the document is a list of filenames from the sample driver showing their role, so I looked them up to see if anybody had mirrored the DDK on Github or elsewhere.
I found that the names are reused from a precursor to the WSS DDK called the Multimedia Device Development Kit (link to a mirror) which contains driver source code for Adlib and Sound Blaster (and the Pioneer 4200 Videodisc Player surprisingly) for Win 3.1.
By the same method, I found this Introduction to Multimedia Drivers document which comes from the Windows NT DDK, indicating that it has sample drivers including source code for Pro Audio Spectrum 16, Sound Blaster and Windows Sound System.
As linked above, the Windows NT DDKs are a lot easier to find, and they do indeed contain WSS drivers for the AD1848 codec, complete with notes and warnings about when to use the enhanced modes. I'm looking through the rest of the disc to see if there's help files specifically about the card, but this might be as good as it gets. I doubt there's much more about it in the manual (if there was one).