It's possible that the original recordings use a custom format, which was the case with many DOS games. I faced the same problem when extracting music files of Tronic which used "DMF" (all references of this format on the internet do not belong to the same one used in Tronic, which was a completely unrelated, custom-made format). Changing bytes in the files changed tunes in-game, which means they're primitive 8-bit recordings, just represented differently from the mainstream MOD, S3M, XM, etc. formats.
To decode such files you would need to have some good understanding of audio encoding algorithms and data representation, as well as reverse-engineer the game.