I guess I didn't understand the rating of the car relay. I thought it meant that the coil would switch at 12v. (And thought, but must have been mistaken, to have read that it was rated for 120v.)
As for the question of why not just open up the recorder and insert the relay after the current gets transformed to a lower voltage, I don't actually own a tape recorder, so I would borrow one, and I'm not comfortable hacking into such a thing.
I can't think how I would manage the power supply (short of buying batteries) if I wasn't plugging it into a wall outlet.
But what I really have in mind is a contact trigger (for said treasure hunt) being used to play a short audio recording when a person reaches a certain point. If you can advise me on a better device for playing such a recording, I would love to hear it. The issue is that I'd like to play it loudly, so I'll need some sort of amplifier, and I'd like to have good fidelity (can't just rip the device out of a record-your-own-greeting.
Moreover, I would rather not use an ordinary AC adapter and then put a low voltage relay on the DC end because the adapter would be drawing power all the while it was plugged in.
Could you explain what a 'mains rated' relay is?