just setup remote desktop
I dont know what you will be doing on your PC that you need sound for so this may or may not be the solution you are looking for.
If your PC is able to be setup for remote desktop (win xp PRO, not sure which versions of vista) you can download remote desktop connection for mac from microsoft's website. Then in the program's preferences on your mac there is a tab for sound, where you can choose where to play the sound from the remote computer (your PC), and one of the options is to play it on your mac. The sound should be okay, since you are on ethernet connection, but you will have to control your PC from remote desktop on your mac, as it will not let you control from both (as with a VNC connection that your mac has, and is included in 10.5).
Another option is to install a VNC server on your PC, then you would not be forced to only use the remote desktop program to control your pc.