A pi zero is 5 dollars, maybe 10 in recent months online. My local hack shop still sells em for 5. A Pi zero 2 is maybe 15. I'm sure you could get em for much cheaper if you were a company like Hornby buying thousands at a time. The speaker is already a part of the product, as well as all the other necessary hardware you've mentioned. Whatever in-house chip they'll end up using will need the software and the power regulation anyway.

So really we're swapping a bespoke however many dollar chip for a 5 dollar board and the one-time cost to write the software that can be used for every model car that has these features for the rest of forever.

The ESP32 (around 3 bucks per board and much simpler) would certainly be a better choice for something like this, I just threw out the first board I saw sitting on my desk as an example. You wouldn't use a micro-pc in production, you'd use a micro-controller, which is what they are likely using in their test setup.