For the record, do you have a photo of the component side of the board?
Curious if there are markings, labels, or whatever showing when it was made. There may also be printing showing programming pins to re-program the sound. That is, decades ago there was a period of years where the best bang-for-the-buck for sound modules used a rather clever analog memory technology popularized by ISD. You'd buy a sound module capable of 30 sec, 60 sec, etc.; I'm pretty sure even DigiKey sold them. If you bought enough units you could have your custom sound pre-recorded. Note the COB technology:
For DIY applications such as a custom greeting card or a one-off layout sound accessory, you'd typically hook up a microphone (or audio source) and press a "record" button to program (or re-program) your sound. The point is this technology is/was inherently re-programmable.
So if this was the technology used, perhaps MTH has a secret programming connector or the like that allows you to re-program the firehouse sound. Again, chasing this down I think would be a fools-errand or snipe hunt given the alternative of inexpensive "modern" digital MP3 sound modules.
