Could be anything. The first thing I'd do, is check to make sure the battery compartment contacts are alright (not bent out of shape and not making electrical contact). Then I'd check the other wires from the main memory unit to
each of the activation buttons to make sure they are connected and one is not shorting-out the circuits, then I'd
check the wires to the speaker itself to see if they are properly connected. If all of your external wires are connected
I would assume something is wrong with the cartridge or the main operating unit itself.
Did the doll get wet, possibly corroding wire connections or the internal electronics of the doll?