If you add another transistor like Q1 connected from Q2's gate to ground you can use it to turn Q2 on and off. Put a resistor in series with the base of the added resistor (no greater than 10 times the value of R1). Connect this resistor to the filter/amp circuit. A high voltage at this resistor will turn off the LEDs. Less than 0.7V will turn the LEDs on.
The spikes it can tolerate are determined by the voltage ratings of Q1 and Q2. To be on the safe side add a small resistor in series with the + power and add a large capacitor (100µF or larger) from the circuit side of the resistor to ground. Select the resistor size to generate a 1-2V drop at the LED current level (R = V/I).