I have the exact opposite problem. I can only get sound through the line out or optical out, but not the Firewire sound setup I use in OSX even though I have the windows drivers for it. Must be because of the firewire connection I guess.
I would suggest downloading directly from Intel the 5400 series chipset drivers. In fact I don't use bootcamp at all. All of the drivers needed for Windows are available directly in the latest versions for each piece of hardware. Even if you install windows from scratch, you just point the install to the intel chipset drivers folder and select all of them. I even installed Win7 this way along side Vista on the same drive. All hardware is recognize on my MP.
I would suggest downloading directly from Intel the 5400 series chipset drivers. In fact I don't use bootcamp at all. All of the drivers needed for Windows are available directly in the latest versions for each piece of hardware. Even if you install windows from scratch, you just point the install to the intel chipset drivers folder and select all of them. I even installed Win7 this way along side Vista on the same drive. All hardware is recognize on my MP.