Audio Compression Manager (ACM). Microsoft ACM is part of the Windows OS. It enables third-party applications to read and compress wave files in many compression formats, such as TrueSpeech and GSM 6.10. For a specific compression to be accessible in audio application, its codec (COmpressor DECompressor) needs to be installed and enabled. To see what codecs you have installed, go to Control Panel > Multimedia > Advanced > Audio Compression.