Some ideas that come to mind include:
beat detection- trigger lighting effects, build a set of turntables that beat match themselves, or make a robot that dances along with the music you play for it
amplitude detection- make a simple vu meter with LEDS
frequency analysis- you could make a project that reacts to different frequencies in different ways, recognizes certain melodies, turns audio into MIDI data, or translates incoming frequencies into square waves with the tone() library
digital effects boxes/digital signal processing- check out what I did with my vocal effects box (all processing done with Arduino), lots of possibilities here: pitch bending, distortion, sampling, delay, reverb, granular synthesis, mixing, and much more... I've provided code in this Instructable that lets you sample at up to 38.5kHz. Here is another instructable describing how to set up a simple audio out circuit with Arduino.
digital recorder- with the addition of an SD card of course (the Arduino has very limited memory by itself), this opens up the possibility of looping large samples and doing lots of other digital manipulations to pieces of stored audio The circuits and code provided here are compatible with SD card shields that communicate via SPI.
graphical representations of sound- Arduino oscilloscope/visualizer
Feel free to use any of the info in this Instructable to put together an amazing project for the DIY Audio Contest! We're giving away an HDTV, some DSLR cameras, and tons of other great stuff! The contest closes Nov 26.
(x1) Microphone Radioshack 33-3038
(x1) TL072 Digikey 296-14997-5-ND or TL082 Digikey 296-1780-5-ND (TL081/TL071 are fine too) I used a tl082 in my examples
(x2) 9V battery
(x2) 9V battery snap connector Radioshack 270-324
(x1) mono audio jack 1/4" Radioshack 274-340 or Radioshack 274-252 or 1/8" Radioshack 274-333 or Radioshack 274-251
(x1) LED Digikey C513A-WSN-CV0Y0151-ND
(x1) 10kOhm potentiometer linear Digikey 987-1301-ND
(x3) 100kOhm 1/4watt resistors Digikey CF14JT100KCT-ND
(x1) 10uF electrolytic capacitor Digikey P5134-ND
(x1) 47nF ceramic capacitor Digikey P4307-ND
(x1) Arduino Uno (Duemilanove is fine too) Sparkfun DEV-09950
22 gauge wire
Step 1: Preparing audio signals for Arduino
When we look at an audio signal with an oscilloscope, we see a similar picture (fig 3). Notice how the audio signal in fig 3 oscillates around a center voltage of 0V; this is typical of audio signals. The amplitude of an audio signal is the distance between its center voltage and its high or low peak. The amplitude of the wave in fig 3 is 2V: it reaches a maximum voltage of +2V and a minimum voltage of -2V. This is a problem if we want to measure the audio signal with one of the Arduino's analog inputs because the Arduino can only measure voltages between 0 and 5V. If we tried to measure the negative voltages in the signal from fig 3, the Arduino would read only 0V and we would end up clipping the bottom of the signal. In this Instructable I'll show you how you can amplify and offset audio signals so that they fall within this 0-5V range. Ideally you want a signal with an amplitude of 2.5V that oscillates around 2.5V (like in fig 7) so that its min voltage is 0V and its max voltage is 5V (see the calculations below).
Min voltage = Center Voltage - Amplitude
Min voltage = 2.5V - 2.5V = 0V
Max Voltage = Center Voltage + Amplitude
Max Voltage = 2.5V + 2.5V = 5V
Fig 4 shows the signal coming straight out of the microphone on an oscilloscope. The signal is relatively weak, with an amplitude of only 200mV, you may find that signals from other sources (ipods, guitars, record players...) also produce audio signals with small amplitudes. These signals need to be amplified to get them up to the amplitude we want (2.5V). Amplification means increasing the amplitude (distance between the center point and max or min) of a signal. Amplification also buffers the audio source (in my case this was a microphone) from any loads that you may put on it later in the circuit, which is a good thing because it prevents distortion.
Fig 5 shows the same microphone signal after amplification, you can see how the height of the peaks has increased so that the wave has an amplitude of 2.5V. But since the center voltage of the wave is still 0, the wave is oscillating between -2.5 and +2.5V. It will need to be DC offset to correct this. DC offset means changing the center voltage that the wave oscillates around (the average voltage of the wave). Fig 6 shows the signal after it has been DC offset; it still has an amplitude of 2.5V, but the center voltage is 2.5V instead of 0V, so the wave never drops down below 0V. (Note- the slight change in shape between the signals in figures 5 and 6 is dues to changes in my voice between the two pics, it has nothing to do with the circuit). The signal in fig 6 is ready to go to an Arduino analog input pin.