After finding some neat cardboard boxes, I wanted to make a small stereo amplifier for listening to musik from an mp3 player. Two 2W 8ohm speakers were used (inserted into two of the boxes), and for the third, a stereo amplifier was to be constructed. Since I did not know what to do with the front of the amplifier box, I realized that I had a lot of green LEDs laying around (which fit the green color of the boxes), and so I decided to make a circuit for lighting the LEDs up in time with the bass notes in the music.
The circuit is fairly simple:
The buffered left/right amplifier inputs are summed in opamp U6. There is a gain of about +10 to make the circuit more sensitive. The signal is then passed through a first order lowpass filter with 
Note: In retrospect, I never calculated the value of the low pass filter while experimenting on the breadboard, and 30Hz honestly sounds much too low. I would say most signal energy from kick drum and bass guitar is around 50-100Hz, so a filter with cutoff at 100Hz would have been better. The OPamp could probably have had a lower gain then, maybe only +2.
Opamp U7 is a voltage follower, but the diode D1 makes it so that it can only charge the capacitor C7 to below the Vref node, but it can never discharge it (since the diode only allows current to be drawn into the opamp output). The C7/R10 circuit is set to have a decay that is fairly long (on the order of half a second) so that once a bass hit comes through, C7 is charged to some voltage below Vref, and is then slowly discharged through R10 and R16/Q3. This is to make the hits light up the LEDs for a longer time and to fade the LEDs out, to avoid a stuttering blinking behavior.
Since the signal is around the virtual ground Vref and the driving transistor Q2 for the LEDs is at ground, a PNP transistor Q3 is used to drive Q2. Whenever a hit charges C7 higher than the base voltage of Q3 it will start supplying Q2’s base with current, lighting up the LEDs.