Thanks guys, I'm looking at all these options.
Jackson, I'm not sure I trust the db rating any more than I trust an ad for a 5000mah 18650, which is why I'm asking for personal experience suggestions. Thanks though, all those options out there mean I should be able to find something good...
dragger, did you have a link or a name for me? I think you're referring to the cheap low-voltage battery alarms. I am thinking of stealing the buzzers off them and using them directly, and yes, I've strapped these on quads in the past that didn't have any sensing onboard. They are quite effective.
mozquito, I'm just looking into the details on the dronekeeper, I remember reading about it. Hope I can find its components, thanks.