good question on a microphone.
there's a number of camera's that have integrated microphones as well , and the signal can be broadcast along with the video.
here's one for instance
http://www.amazon.com/SC2000-540TVL-Cam ... B00F5Q4FL4
But if you've ever used one of these low voltage alarms, they are **** LOUD and rather annoying, and can be heard for quite a distance.
Most of these alarms are set to go off as a warning to "land quickly" as well, (and believe me, flying with a buzzer blaring loud like that, it makes you want to land as soon as possible to make it stop!).
So in theory, you don't want to be so far out of range that you can't hear the buzzer in the first place.
theoretically you'd want to have it go off only when you're close to home and about to land, well within hearing (and visual) distance. - or even as a last minute warning prior to 2nd protection voltage kicks in.
setting it to 3.5v or if you're a conservative flyer, maybe 3.6 per cell., depending on what your 2nd level battery protection is set at.