ACM (InterPlay ACM Audio) is a special compressed audio file format extensively used for storing sound effects and human speech for most of the 2D role-playing game titles published by InterPlay Entertainment in the late 1990s and early 2000s. Some of these games include Fallout, Baldur's Gate, Icewind Dale and Planescape Torment. While the encoding scheme used in ACM files doesn't achieve that good compression ratio compared to MP3, Vorbis or any other of the popular audio compression formats, ACM files require less system resources for decoding, which made them an optimal choice for the games from that period, which were pushing the computer hardware to its limits for the time.
Although there is no official statement to confirm that, it's assumed that the ACM format was originally developed by BioWare, since all of the games using it were based on BioWare's Infinity Game Engine. A typical ACM file consists of two main parts - a short header, containing important metadata, and an ACM-Stream, containing packed data blocks. The header starts with a special ACM-Signature defining the exact version of the ACM file, followed by information about the total number of samples and channels stored in the file and its bitrate. Although in theory these parameters may vary, in practice all ACM files are either 16-bit mono or 16-bit stereo at 22050Hz. The ACM-Stream section consists of special data structures called BitBlocks that contain the encoded audio data. BitBlocks can have a variable length and can be unpacked into a variable number of samples, which, when combined together, reconstruct the encoded raw audio data.
ACM files are generally not natively supported by media players. However, there are a number of applications developed such as ACM2WAV and ACMPlayer for Windows and libacm for Linux, which allow playback and conversion of ACM files to more common formats, such as WAV.