While experimenting with Tsukihime's music, I discovered that WAV files are compatible! I tried compressed lossless formats like FLAC and WavPack, but they didn't work.
Either way, the point is, why doesn't the README state this? There are several distinct advantages WAV has over OGG vorbis and MP3.
For the majority of the not-so-tech-savvy public, getting audio into the WAV format is much easier than OGG vorbis, and usually even easier than MP3 since WAV is the default format for CD ripping 99.99% of the time. Then there's the audiophile angle where WAV is the only compatible format that actually give the full CD-audio quality. Also, WAV, being uncompressed linear PCM, uses the least amount of CPU which can be useful for virtualized environments (especially since the recommended format, OGG vorbis, use the most CPU out of the 3)
Did the mirror moon staff themselves not know that WAV worked or something?