Soundwave is ...
A visualization of the fourier transform applied to sound data. Analyzes a time-domain sound sample using the Fourier Transform, and graphically displays the results and sound in real time. Attempts to find the dominant pitch being played.
This program is still in a very early development stage. It can do some things, but it's not very good at them yet. It's written in Java so (theoretically) any OS with a JVM should be able to run it.
Most of the usage information is included in the distribution, so I'll just quickly sum up what it does. Soundwave takes a fourier transform of a very short (a few milliseconds) clip of your audio file. That converts the time-domain data to frequency domain data. The data is then converted into a colored stripe with each pixel representing a different frequency, and the color of the pixel representing the amplitude of the frequency. This is done many times and is scrolled across the screen.
The result is a graph of time versus frequency versus amplitude, and when scaled correctly this graph resembles actual sheet music. Of course there tends to be a lot of noise and consequently the graph "sheet music" is rather difficult to read at times, but then again this is a work in progress.