Beyond Good & Evil PC Audio by BtEO at 6:50 PM EDT on April 5, 2010
vgmstream already has support for the audio files from the Gamecube version of the game, and they're reported as GC 4-bit DSP with a WAA,WAC,WAD,WAM header.
The files extracted from the PC version also follow that pattern, but they don't play with the latest vgmstream. One of the extractors I have for the sally_clean.bf archive will helpfully offer to output WAVs instead of the actual files, but this loses looping information.
I'm guessing the header is mainly to add looping support, and that each of the various ports wrapped that header around a different file type. Vgmstream currently just appears to assume it will always be Gamecube DSP. It would be nice if that could be expanded.