Admittedly, a somewhat esoteric topic/tip, but I ran into this recently, so I figured there may be other folks who can benefit...
I've got a Dell Latitude D820 laptop, running 64-bit Windows Vista Ultimate, and I've occasionally wanted to be able to use my laptop as a speaker system for my Zune when I'm on the road, but hadn't been able to figure out how until recently.
One of my colleagues gave me a 1/8" stereo plug extension cord (1/8" stereo on both ends) which was perfect for plugging the Zune into the line-in on the laptop. Unfortunately, I quickly discovered that there isn't any way by default using the SigmaTel drivers to monitor the input from the line-in.
Fortunately, a quick search came up with this post on dellcommunity.com:
The registry hack posted (see warning below) worked a treat for me, allowing me to enable monitoring the line-in by modifying the settings in the Playback properties of the Sound control panel applet:
The registry setting enables the Input Monitor button show above, and when enabled, allows me to playback my Zune over my laptop speakers. Excellent!
WARNING: Never edit the Windows registry if you don't know what you're doing. Editing the registry incorrectly can render your system unbootable, or cause other major problems. If you have any concerns about editing the registry, just DON'T.