I did this a long time ago with a 6V circuit. The pic was running from the battery (with a series diode = 5.4V) but there was a 3.3V regulator elsewhere on the circuit board. I ran a jumper wire from an ADC pin to the 3.3V reference voltage. As the battery voltage begins to drop, the ADC reading will begin to rise. When the ADC reading reached the desired point, the SOUND command alerted you.