Alexander,
You want to google the paper titled "Defining speaker locations in multi-channel streams and WDM audio drivers using the WAVEFORMATEXTENSIBLE structure" by Carsten Schulz to learn a lot about channel mapping as performed by w2k. It's a pity there's nothing like it for xp or w7!
Wine's mmdevapi does not yet behave like native's 5:1 cards. People have sent me the following mmdevapi:render test data on native machines: render.c:182: chan: 6 render.c:183: rate: 48000 render.c:184: align: 24 render.c:191: Mask: 3f
IOW, the system's mixer format is really 6 channels! There presumably, DSound sends stereo and 6 channel data to the mixer. I suppose MS evolved DSound's mixer into mmdevapi's one, with some lessons learned.
BTW, as I'm not aware of a means to extract from ALSA the user's speaker configuration, Wine likely needs either a winecfg setting (like MS' system preferences) or a registry key to advertize the mixer's dwChannelMask (and default to 0x3f with 6 channels).
Regards, Jörg Höhle