So I guess the question to figure out is whether it's a problem with the playback code or a problem with the underlying composition. I was going to suggest checking
Exotica for a raw .mod file, but it seems it's a .cust, presumably meaning playback and composition are inextricably integrated. I think the odds are long, but maybe EaglePlayer or similar can tease out the "correct" sounds? Otherwise this is probably a task for the WHDLoad guys to figure out.