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 format is free, which apparently is one of the reasons for its popularity. The used codecs supports variable bit rates (as a default), and .ogg files usually play back easily on all platforms. All things being equal, the sound quality exceeds that of mp3 somewhat.