A headerless file of Dialogic/OKI ADPCM audio data commonly comes with the extension .vox. This ADPCM data has 12-bit precision packed into only 4-bits.
this is a lossless-format (and encoding algorithm) for Apple platforms. M4A is widely used in iPod players and as a component in QuickTime media-players. The m4a codecs decoder is found in open sources - this allows third party users to open files in that format.