5v Low Battery Buzzer Beeper with servo connector only USD $2.49
Plug this easy buzzer into your fc and setup the voltage limit in cleanflight and have yourself a simple low voltage alarm.
This is as easy as it gets and no more dealing with lost alarms or having them chopped up by props or our worst enemy, the ground.
NOTE: There are 2 types of buzzers available. 5 and 12 volt. you want to be certain you buy a 5volt for a flight controller connection.