can you replace the 'audio byte' that is stored in a pre-recorded musical greeting card? Answered

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