After removing and disassembling the box which appeared to be the sound source, a disconnected wire was found on the piezo speaker.
A second or two from a hot soldering iron put the wire back in place, and the sound was functional again.
The last trick was re-assembling the case. Since the iron was already hot, I just melted the plastic a bit and that seemed to do the trick.
The box was snapped back onto the truck and returned to a very happy 3-year boy.