Bootcamp/Parallels - how to use the the line-out on my MacPro
I don't have an answer for you for Boot Camp, but for Parallels you should be able to (if you're in Coherence mode): 


Click on the Parallels menu (Just the Devices menu if you're not in Coherence)
Go to the Devices &rarr; Sound &rarr; Input Device menu
Choose Built-in Input device or Line in Input Device depending on what shows up.


Example: