Shadertoy exposes audio through a 512x2 texture, where the first row is the spectrum, and second row is wave data.
Pixel format is GL_RED GL_UNSIGNED_BYTE,
meaning that each pixel contains only one 8-bit channel.
According to this shader from iq the audio in the browser is supposed to have a samplerate of 48kHz, but as it turns out, that's not the case: most likely it will be in 44.1kHz.