For the first time recently I ended up with an Electronic Speed Controller that did not have built in Low Voltage Protection but did support LIPO batteries. You should never let a LIPO battery drop below 3V (You should get rid of it if it does.)
As the esc did not have LVP and independent solution was needed. After a little research I found these usually come as a little circuit board with an led screen that connects to the balance charger on a LIPO battery.
A LIPO battery generally has two connectors. One to connect to your model and one to charge the battery and balance the battery cells. The charge connector is not needed when the battery is in your model so you can leave a low voltage protection unit attached to it.
I managed to buy some units off ebay that work perfectly and only cost a few pounds each.
The units are simple enough. You give them a voltage you want them to warn you at and they will buzz when they drop down to it. They work well and I have never had any trouble hearing the buzzer.
Here is a video of them in action. The display even cycles through the voltage of each cell
Shane's Shed 