Hi everybody
Have you considered piezo buzzer's for back up alarm's?
I use a Radio shack 206-2399 in my Mack dumptruck for low oil, air pressure, power divider lockout warning. 3-28volt dc. It does pulse like a back up alarm, higher pitch. If it's too loud, just cover the opening with a bandaid, they make great muffler's for them.
It would make a nice backup alarm for rc model's. I'll have to double check the part # tommorrow. Hope this help's! Later, Neil#2 aka doodlebug.