RIFF signifies a standard Windows RIFF WAV file (check any WAV file format specification for more info), but when used in GrimE the file has been compressed (usually starting from the 8th byte of the data chunk - the actual pcm data) in accordance with the MCMP block. However, if no such file is found in the .WAV file (such as in Phantom Menace and some instances in Grim Fandango), the file is likely to be uncompressed (but may follow a special scheme when dealing with a 12 bit WAV).