This is a small audio visualization program I wrote to teach myself how to use the OpenGL and JACK APIs. The nice tutorial Learn OpenGL was very helpful, as was Khronos's OpenGL wiki. The JACK code was adapted from simple_client.c from the JACK project's example clients repository.
gl_thing continuously copies a few frames of raw audio data from JACK into a buffer. Every time OpenGL renders a frame, it downsamples the data from the JACK buffer into a 1-dimensional OpenGL texture. The vertex shader in vertex.vert uses this texture to offset a highly subdivided line of vertices, effectively drawing an oscilloscope-like graph of the audio waveform on the screen.
You need to install the freeglut and JACK audio connection kit headers. I've tested this using libraries from the Arch Linux and Raspbian repositories.
Simply run make to compile it.
Run ./thing. You might need to use a JACK patchbay to feed it some audio data.
It doesn't have any runtime configuration yet, so have a look in thing.c if you want to change how the graph looks.
If you edit the shader source files, it will attempt to recompile them on-the-fly.