The card I have has 2 unused solder plates.
I actually found a third plate just above the left hand battery.
I would regretfully say that most signs point to no. Regardless of whatever chip is hidden under that blob of gunk, most likely they'll have used a one-time programmable chip, or set the fuses to be read only. However, that doesn't stop you from making your own circuit :D
yeah - its likely cheaper for the builder to use a one time programmable rom for the sound, then seal it all up in epoxy.