AGSC (File Format)
AGSC is the sound format for Metroid Prime and Metroid Prime 2: Echoes. According to the debug maps in Metroid Prime, there are references to various "classes", with one of them controlling audio in the map, known as AudioGroupSet. So presumably, AGSC is an abbreviation for Audio Group Set Class.
The audio codec used for AGSC is the standard Gamecube DSP ADPCM codec, as described in the Gamecube SDK.
Format
There are two types of AGSC files: those found in Metroid Prime, and a slightly different version in Metroid Prime 2.