Just found an old friend again, great audio problem checking tool....

[Level42]

When I worked with ADT about 17 years ago, I had a small tool to hunt for audio on things like phone lines etc.
It was a small speaker unit with a built-in super simple amplifier. I think they hacked it to include two wires with crocodile clamps for easy use for us. It was a great tool to hunt down any audio circuits.

I remembered it's brand, Archer and through the wonders of google I found that it's still being made (although a bit more modern styled):

Remeber to shorten those links!

I even found a European (UK) source for it here:
Another link shortened.

Highly recommended for searching audio problems on your cab.....or anything else really