It looks like MRC makes a sound decoder that might work for DC:
MRC 1818, G/O UNIVERSAL: SD45/MP15
Not just for DCC systems, this decoder is dual mode so Power Pack Users aren’t left out in the cold! DC railroaders can get brilliantly synchronized sounds on their layout as well. And amazingly….. DC power pack users can also enjoy the 28 NMRA function, capability of this G & O gauge decoder when coupled to the unique & exclusive MRC BlackBox (#0001050) control unit.
From the 1818 manual:
The decoder can also be operated by a regular DC power pack. This will give
you synchronized engine sounds only. If you wish to enjoy the full array of
sound functions using your DC power pack, the unique MRC Blackbox (item
#0001050) for DC operation will allow you to control all of the sounds in your
sound equipped locomotives. And, the MRC Blackbox is easy to setup and use.