I've done some further testing.
Server, Win7 x64, Stereo Analog Out Realtek, MC18:
All play fine except .ogg 4.0 or 5.1
Office Desktop, Win 8 x64, HDMI to Receiver, Directsound, MC17:
.ogg is a problem,
.wma is a problem
HTPC, Win7 x64, HDMI to Receiver, WASAPI Event, MC18
.ogg is a problem
All files in question play perfectly fine using whatever other player,
explicitly tested today using VLC, Foobar2000, XBMC
All other surround files play perfectly, flac, dts, ac3
All stereo files play fine, but I'm into Surround
I'll upgrade the office pc to MC18 as a next step to see if wma is
solved by MC18. UPDATE: Nope, still the same.