What is a ADPCM file
ADPCM files contain audio data that is saved in a special way. These files are optimized for storing digitally recorded or sampled voice data. Contrary to the more common WAV files, ADPCM files do not contain a header containing information such as the encoding format or sampling rate. Thus, these values must be known in order to play an ADPCM file properly.
The audio data inside the ADPCM file is compressed into several 4-bit samples. The sampling rate is, commonly, either 6,000 or 8,000 samples per second, but this frequency is not specified in the standard directly. ADPCM stands for Adaptive Differential Pulse-Code Modulation and is a variant of the Differential Pulse-Code Modulation (DPCM). It allows a greater reduction of bandwidth.
Here's a small, but not exhaustive list of programs that can open ADPCM documents: