The locomotive itself will operate on DC, it doesn't care. The 'True Blast' sound board does, since the DC from the transforer is what triggers either the horn or bell when you press the appropriate button.
On the 6 amp reverse board there are two 2-pin connectors. One is a DC output that was intended for connecting a tether to a second locomotive.
The second 2-pin connector passes AC straight through from the track pick-ups. This is the one the 'True Blast' sound unit connects to. When it detects the +/- horn/bell DC voltage on top of the track AC, it triggers.
If you want to operate the loco on DC, you have to unplug the sound board from the reverse unit.
I will second what Len2 mentions about the Williams True Blast 2 units.
The True Blast 2 unit will continually sound off in DC voltage.
The motors are DC but the True Blast 2 needs AC with a DC pulse or something like that to function.
Lee Fritz
