Have you gotten your hands on one? Odds are there’s a COB (a black blob with a chip under it) and not much else inside (I somehow doubt toddlers care about tuning accuracy and there’s enough ultracheap “sound chips” out there to make it unlikely that they use a cheap microcontroller). If there’s nothing else to tweak, playing with the supply voltage may have some impact (or it’ll just produce horrible noises).
I havent got one yet, but i will order one. It will take like 40 days to arrive but im only 12 so i dont want to risk my money on buying them in bulk just to see if they have any proper parts that can be circuit bent.
for this kind of thing it is better to do the secondhand trades to find older keyboard toys, as said @fredrik it is surely a black blob inside a modern toy like this one, while in the old there is resistance for the speed of sound, which can easily be replaced by a pot
presumably one wants several of them, and finding them one at a time in thrift stores may not be feasible. With Furbies at least presumably enough of them were made and sold that one can find 48 of them with enough determination and cash, but not so much random toy keyboards unless a pallet of them turns up in a forgotten corner of a warehouse.
So the challenge is to find something hackable but available at low cost in quantities of (however many)… not so easy.