Trixter, digital P.I. and archaeologist, has been tracking down authors who long ago reverse engineered custom audio formats. He stalked down one Adrienne Cousins, the author of a program called Sputter, a multi-format audio file encoder/decoder… all in 16-bit x86 ASM! The source includes codecs for the Covox ADPCM formats mentioned recently. Thanks to Kostya, though, for independently reverse engineering and documenting the format.
Still, there might be some other gems in the Sputter source code. The author has granted permission to redistribute the source, and it is now in this directory: http://samples.mplayerhq.hu/A-codecs/CreativeADPCM8bit/. Also in that directory is a package called VEDIT.rar which contains an old EGA-based program that is able to encode VOC files to that arcane 2.6-bit ADPCM format that the Sound Blaster could encode and decode in hardware. If reversed engineered, it might describe how the 2.6-bit algorithm works.