in_dsp is no longer under development. See in_cube for additional work along these lines.
in_dsp.dll is a Winamp plugin created by Destop to play a certain format in which music is stored by Nintendo Gamecube games, originally debuted on Dextrose.
I've made a few modifications which make it usable with some files with different headers, stereo interleaving, and looping. It isn't perfect... but it's noticeably more perfect than it used to be.
in_dsp.dll v1.9, state of the art as of 11/15/05. Also, source. The original source is here.
This version supports the DSP files used in: DSP files can be found trivially on a Gamecube disk, usually in a directory with a name suggesting music ("BGM", "stream", "score").
I've documented the various header formats I've seen here.
Several games (specifically those HAL had a hand in) contain SSM files, which are collections of DSP files. SSMEx (v0.1) will extract those files. I don't have the algorithm exactly right yet, but this produces usable files.
Additionally I've put together, on request, a program to convert the standard mono DSP files created by the devkit DSPADPCM.exe into stereo (either by doubling a mono file or using one mono file for each channel) files for Donkey Konga or Star Fox Assault. It's been tested on a Gamecube with Japanese Donkey Konga 2.
dkdsp v0.4.
Notice at the point of this program is to generate new DSP files for use in Donkey Konga. It is not necessary to combine right and left channel DSPs for playback with in_dsp, if you choose either it will load the other as well.
For info on decoding another major type of streamed audio used on the Gamecube see CH3's Cube2wav, which includes source.