I would love to setup a sound reactive input on my arduino, aka if there is a beat high enough, the signal goes high/low. Ive done some googleing around and couldnt find anything. Would just a simple audio amp do? could you accomplish this somehow with a transistor? op amp?
my knowledge is limited, but an audio amp would work, possibly just a preamp.
i would look at a lm386 based amplifier, however you could also look at using a transistor as an amplifier they can certainly do that. then just read the voltage on the analog pins, you will get a value between 0 and 1024 that corresponds to 0-5v, you can set the threshold manually to turn the led off and on, similarly you could use pwm and pulse the led brightness accordingly :D
There is a thread here http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1202056662/0 that covers a more complex application then you are looking for, but why not ask there for more info on the amplifier circuit he used to get microphone input detected by the arduino