WAVE file

Waveform Audio File Format, or WAVE, is an audio file format developed by Microsoft, and is the primary audio file format for use by Windows PCs.

Most Tt-developed games use the WAVE format on PC in some manner or another. Different file extensions are used depending upon the content and use-case. Early games decompressed the music files during installation; these decompressed files use the .WGT extension. Most games use uncompressed WAVE files for sound effects and sometimes voice grunts; these use the standard .WAV extension. Later games add support for ADPCM-compressed WAVE files; these use the .ADP extension to differentiate them from uncompressed files.

Technical Documentation
As this is a format with publicly available documentation, further documentation can be found on Wikipedia.

To encode an ADP file using FFmpeg, use the following command line input: ffmpeg -i INPUT_FILE.wav -f wav -acodec adpcm_ms OUTPUT_FILE.ADP