Hello ya'll!
I have a question regarding Arduino and it's capabilities in receiving different sound frequencies!
I want to build a thingie that receives different notes (e.g. the note E from a guitar), and then be able to with the gathered data to make different leds blink depending on the note played.
Is this in theory difficult to make? What else would I need besides the Arduino?
Thanks!
This is harder than it would first appear. A guitar does not output a perfect sine wave at the primary frequency. Instead it's a complex waveform with a primary frequency and several harmonics all with attack, sustain, etc. There are ways to find the principle frequency: fourier transforms. Fourier transform - Wikipedia
Be sure you are sitting down before you visit that wiki page... It's math soup.
A small microcontroller such as the Arduino is unlikely to have enough CPU power to run an FFT (Fast Fourier Transform). But, you might be able to find another way to determine he frequency of an audio signal applied to an analog input. Another option would be to use external hardware to act as, say, a frequency-to-voltage converter.
Apart from the Arduino, you'll need an amplifier of some sort. The Arduino needs a signal of 0 Volts to 5 Volts (the signal must never be negative, nor exceed 5V, or the chip will be damaged). An op-amp (operational amplifier) circuit is commonly used.
This guy has done a similar project and it wasn't easy:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234487715/4#4
Ok. Thanks for the help!
Quitting this project because it's just a tad above my knowledge and timeframe ! 
What about this:
A Box with a couple of leds. Ad knob controls the BPM of the led. You have on input source (a 3.5mm phones input) where you can plug in an instrument or musik etc. You now have to match the given bpm with your instrument...
This seems a bit easier to make. Thanks a´for any sort of help!