How to make LED's flash to music at low volumes?

I currently using a tip31 transistor to make LED's pulse to music. However, currently they only start to flash when the music is turned up past the half-way mark on my iphone. The LED's are a pre-wired strip of 48 LEDs that requires 12v to operate, therefore the transistor uses the music to turn on and off the 12v supply. Could somebody recommend an opamp circuit, a more suitable transistor or another method to make them also flash at low volumes. Thanks