AGSC (File Format)
AGSC is the sound format for Metroid Prime and Metroid Prime 2: Echoes. According to files found in the Metroid Prime 3 E3 2006 Demo, the "AG" stands for Audio Group, so presumably "AGSC" is an abbreviation for Audio Group Sound Content or Audio Group Sound Collection. The actual audio data is the standard Gamecube DSP ADPCM codec.
Format
There are two types of AGSC files: those found in Metroid Prime, and a slightly different version in Metroid Prime 2.