In this post, the Audigy RX PCI-E soundcard was confirmed to work in pure dos via the MPX Play program:

SB Audigy RX ms-dos compatability?

It is essentially a PCI card with a PCIE bridge chip to make it work on the latest and greatest mobos.

This PLX Technologies bridge chip is fully transparent to an unaware OS like DOS, which means traditional PCI addresses and stuff will work on the PCIE bus.

Audigy SE uses the CA-0106 processor.
Audigy 2 uses CA0102 to access CA0151
Audigy RX uses CA10300 from Audigy 4
The CA10300 is simply an unleaded version of the CA0108

Therefore the RX should work very similarly to the Audigy 2.

What would entail modding the modded Audigy 2 DOS drivers to make the RX initialize in Pure DOS mode?

In that same post, someone entertained the idea of patching in the PCI bus addresses of the RX to the DOS drivers set.

I have never done this kind of thing and I wonder how hard it would be?

If this could work, this could make a very interesting machine based on modern hardware have hardware sound support for old DOS games.