I am asked regularly if I can make a sound car that works on DC voltage as opposed to DCC.  In summary, not well.  The concept is built off of existing DCC technology and is therefore only as backward-compatible as any other DCC product.

Here are a couple of demonstrations of sound cars on DC.  The biggest challenge with DC is the fact that the DCC sound decoder requires 4V before the sound starts.  This means the loco is moving pretty fast before you get any sound.

I have discussed this with the manufacturer recently and there really is no way around this threshold due to the requirements of the circuit.

The only work-around I can come up with for DC would be to place a resistor in the DC motor circuit such that it would require 4V to start also.  The only down side to this would be you would either need to accept that the one loco (or consist) ran slower and rquired more voltage to operate or you would need to modify the whole fleet if you wanted them to respond identically.  All of this depends on your needs and what would work best for your application.

There are a handful of DC-based sound platforms on the market, including options from MRC, MTH, QSI, and BLI, that may be adaptable but I do not own them and therefore cannot test the concept.

I do have an MRC Black Box that connects in the same way as many of these products.  This product is essentially a single cab DCC controller.  It converts the output of a conventional DC cab into a DCC-compliant signal but does not allow the selection of individual locomotive addresses.  This means everthing on the track responds to it's commands (the same as DC).  The advantage though is that it is a cost-effective controller that allows you to access DCC sound and lighting functions.  It WILL NOT control a DC locomotive and may damage the motor if it is left on the track for an extended period of time.  It will certainly control a sound car and most sound-equipped locomotives but it requires all locomotives on the track to be DCC compatible. 

Here is the Digitrax version…

http://youtu.be/pICpR1Mb4Ag

Here is the soundtraxx version…

http://youtu.be/WAJxEaCjQ68

The advantage of the Soundtraxx version is that the horn can be activated by a quick turn of the throttle.