I have explicitly shared the the folder on D: where the music resides. I've also renamed that folder to Shared Documents\Shared Music.
Then I told WMP to explicitly monitor that folder—getting all excited last night thinking that was what I was missing. Since there are thousands of tracks residing there and it was going to take awhile, I went to bed. This morning I checked the Yamaha and, no, it still doesn't see these tracks.
Once again, I'm counting on someone herein to know something, in that Yamaha's support is S-L-O-W in coming and when they finally do respond, it's usually in, uh, Engrish. To date, their responses are also incomplete. This really bothers me, in that it's a key feature AFAIC, and they should have included some specific info in the manual, or at least have it posted in a FAQ at their website.