What did the "gear warning horn" on the North American P-51 sound like?
Interesting question.  Unfortunately, I couldn't locate the part in the P-51D/K parts catalog, but, if an E-bay seller is to be believed, it had an E-2 Faraday horn, which appears to be an electrically actuated alarm.  Faraday is (was) the name of an alarm manufacturer most commonly known for their fire alarms.  Based on a history paraphrased from cteksys.net (emphasis is mine), this seems to be plausible:

Faraday, was founded in 1875 and is now one of the leading
manufacturers of Notification Appliances, Fire Alarm Control Panels,
and Clock Systems in the United States.  (Statement in 2002) ...
... In the 1940s Documents have been located with the name Schwarze-Faraday at 130 Church Street Adrian Michigan showing the
company pressing into signaling equipment for industrial applications
including factories , ships, and airplanes.

It's not the original, but a circa-1960s Sperti-Faraday horn (the company name from 1937-1965) sounds like this: