He's deep into dinosaurs right now, so the next small project turned into this: a Toniebox that plays whatever I want, wearing a figure that never came from Tonies at all.
A Toniebox doesn't know or care what a figure looks like — it reads a chip hidden in the base and looks up what to play on its own SD card. Officially, the only chips it recognizes are ones Boxine registered at the factory. Once you can read and write that card yourself, that stops being true.
teddy, specifically the TeddyBench tool inside it, is what actually does this: pull the card out of the box, dump it, and TeddyBench shows exactly which file plays for which chip. Swap the audio, point a spare chip at something new, write the card back, and put it back in the box — no server, no ongoing connection, it just plays from then on like any other Tonie.
Update: TonieToolbox looks like a more modern take on the same idea — still actively maintained as of writing this. Haven't switched over myself, but it's worth a look if you're starting fresh.
Five things, none of them from Tonies.
A flat black disc to build on. Natoons figures already come glued to one.
The chip the content gets tied to on the card. Has to be the right type or the box won't read it.
Glued under the base so it snaps onto the Toniebox's own magnetic mount, same as the real ones.
Once a figure's sound is set, this seals the magnet and chip in for good.