Hugh, yeah, when I was typing up the synopsis, I was thinking much the same. It doesn't make sense to me that I can hear all of the other voices although here's a hint, I have the other voices set up to use the Microsoft GS Wave Table Synth in the mixer for playback because the Sibelius Player does nothing. When I click on the "test sound" button in the mixer for the individual voices, I get playback with that setting AND with the VDL battery parts (via Kontakt Player). Don't know if that helps at all.
One more thing, I just tried the keyboard parts as well, Kontakt Player too and again, not coming through the ear buds. Interesting.
VDL Standalone? I'm not sure how to check that but if you give me a hint, I'll certainly try it.
Steve