A quick summary of this obscure format: .WOW is a strange .MOD variant associated with the Mod's Grave player for DOS. The only thing particularly notable about it is that it has 8 channels but uses the M.K. signature, meaning it's a false positive concern when loading regular .MODs. The only program I can find that creates these files is a 669 converter packaged with Mod's Grave. The documentation for this program is in German but (a bad Google Translate job) suggests that this player is the sole purpose of the .WOW format.

I converted every .669 I have to .WOW while adding support for this format to MikMod and found some odd (but valid) .WOW files that trick OpenMPT (attached).

1) "ACIDFU~1.WOW" and "PURPLE~1.WOW" have an extra byte added by the converter for some reason (easy fix, just clear bit 0 of the result of file.GetLength() in this check).

https://github.com/OpenMPT/openmpt/blob/master/soundlib/Load_mod.cpp#L553

2) "DARKMO~1.WOW", "into the maelstorm.WOW", "MEXIKA~1.WOW", and "THELUN~1.WOW" rely on counting length word=1 samples in the file length check to be correctly identified as WOW, which libopenmpt does not (sample length is counted after length=1 samples are pruned).

https://github.com/OpenMPT/openmpt/blob/master/soundlib/Load_mod.cpp#L836

Fixing the latter of those two bugs introduces another problem, which is that this .MOD on Mod Archive gets detected as a false positive .WOW (this happens with both libxmp and libmodplug).

https://modarchive.org/index.php?request=view_by_moduleid&query=129410

Fortunately there are some more checks that can be added based on quirks of the converter: 1) the restart byte is always 0x00; 2) for every non-zero length sample (including length 1), the finetune byte is 0x00 and the volume byte is 0x40 (669 doesn't support either so these are just defaults). Filtering on either of these quirks prevents the false positive from ponylips.mod. I ran a check on every .MOD file from Mod Archive and couldn't find any evidence of other potential false positives (at least on Mod Archive). These checks both assume 6692WOW is the only source of .WOW files but that seems like a safe assumption.