How to wire in my TIP120
The problem is the voltage taken by the blue flashing LED. Depending upon the make this should be around 24V for a 24" strip.



The circuit shows a very simple sound operated transistor switch. 

C1 ensures only AC is fed into the circuit. R1 and VR1 form a potential divider to reduce the size of the voltage to a suitable level. VR1 allows you to set a sound (output) level that will turn the transistor on.

The base emitter junctions of the transistor rectify the positive half cycle of the AC signal. D1 prevents the megative half cycle appearing across the base emitter junction and provides a current path for the capacitor.

For the transistor to be turned on the base needs 1.2 Volts. With the values given the minimum speaker voltage output needs to be around 2.4V. (VR1 = 10k)
VOX switch (more gain + smoothing/delay)



Q1 (+ D1) rectifies the AC signal. This switches Q1 on and charges C2 to 12V providing a current to the base of Q2 to turn it on. C2 smooths any small drop outs in signal. When the signal drops it holds Q2 on for a little longer.

I've put in a 'test' circuit (in blue) so that you can make sure Q2 / LED strip is working. You don't need to add this bit if you don't want to. 

Start with 12V to see if it works with this voltage.