Looking at the sound code, the subsystem can be divided into 4 layers:
In this entry, I'll be dealing with 2 1/3 of these layers!
The sound subsystem is supposed to be OS agnostic, with the majority of the code being reusable (with macros for some of the OS-specific interfacing, like dealing with waiting). "configure.c" is the configure script for Linux sound. Yes, a configuration script written in C. It was a barbaric time. "soundcard.c" is the entry point for Linux, and "os.h" provides an abstraction layer for the rest of the code.
The main switching point, as it were, is "sound_switch.c". It implements the status device in a mildly nutty way - there's a fixed, shared buffer for the status message, so only one person can have it open at a time. For everything else, it delegates, calling functions audio_*, sequencer_*, MIDIbuf_* and mixer_devs* for the per-device-type operations.
In the third layer, "audio.c" handles the generic playing of samples, delegating in turn to the specific drivers (which I haven't read) through "dev_table.c". "dmabuf.c" handles DMA in a generic way, called from the specific drivers.
And that's where I've got to so far. Next up, I plan to poke around the other abstract device types, and then look at the drivers for a simple sound card.