Audioengr,
I believe you are wrong on this. If you follow the second link, we did a little experiment.
Try the following...
"Disable the sound card. Reboot to safe mode. Delete kmixer.sys to recycle bin so you can restore the file later (need to boot to safemode to delete the file though). Reboot PC then run itunes. See if you can use itunes to play to the remote speakers. This will prove that whether or not the kmixer will do anything to the audio stream. Remember to restore kmixer and re-enable the sound card when done."
and guess what, it did play sound through Airport Express.
So, your claim "iTunes with a PC does not bypass Windows KMIXER" is wrong.
Regards
Mhfun