I know little about WMP. But one guess is your output device is now set up for hi rez, maybe because it is set to 16/44 in shared mode and that is limiting you. I would start by calling up Control Panel, Sound and select your output device. Hit Advanced. Check the 2 exclusive mode boxes. Then, select 24/96 from the pulldown and test to see if you can play 24/96. You may also want to select your output device and hit Configure. Some devices have you set allowed output characteristics.
HDTRACKS sample rate
I downloaded the free sample from HD TRACKS, which has a sample rate of 24/96 to see what the fuss is about. When playing it from my laptop to my Marantz SA8004 USB input the sample rate reads 16/44.1 The Marantz is supposed to accept inputs up to 24/192. Anyone know why it's only playing at the sample rate of 44.1?