As I thought, the problem is in XMPlay, not your plugin. I thought that would have to be what's going on, since other players have no problems with it.
The samples certainly are used in the patterns (at least according to MPT...), both the AM/FM samples and the regular ones.
I agree that if it uses synthesised samples, it should have the other .nt or .as file in the archive with it, but after trying to download it from 5 different sites, none have the additional files =( So yeah, I'd say that 'corrupt' is the right way to describe a file with missing information...
How other players and even previous versions of XMPlay managed is still a mystery to me. Maybe something Ian will look into before he releases 3.5 - it seems that a lot of things 'broke' during the transition from 3.3. to 3.4