The Dell port replicators -- as far as I know -- do not contain their own audio cards. Instead, they work as extensions of the laptops on board sound hardware. Many of those built-in devices have outputs for s/pdif and other options on the sound chip, but you can't access them without the docking station since there's no room for the physical ports on the laptop itself.
My old Dell "brick" uses the (mostly awful) Maestro2 chip. I had a bit of trouble getting the s/pdif to work at all at first because there's multiple drivers for this card and not all of them support the s/pdif output. When I finally found the right driver, the only new control available was an "advanced" button added below the main mix output level control. This brought up a dialog with some checkboxes to enable or disable the s/pdif. I don't remember anything that allowed you to set the sample rate, though.
I think Rozzer makes a good suggestion. There's just too many automatic selections made in that OS that you can't directly access.
I'm so glad I ditched Winblowz....
