I spent ages sorting out my case, unfortuatly most cases and m/b are incompatible with each other, which is why the front panel is often left unwired even on shop sold machines.
I might be able to help you out but to give you a definate answer I will need name of case and m/b (with website address so I can see them).
Photocopys or online manual of the relivent pages of the instruction book (diagrams & instructions if avalible).
Just by looking at what you have written I wouldn't like to give you a 100% answer although it loooks to me like:
port_L and R -> spk out L and R
Mic bias -> ??? sometimes GND, possibly presence
Return L, and R -> GND
Ground -> GND - pretty sure about this one
Mic in -> ???
Although I wouldn't do any of that without me having a look at the stuff I requested.
On the other side, generally speakers etc are low voltage so you can do these by trial and error without concern over whether your stuff will break or not.
One of my friends did it this way and nothing went wrong.
On a side note a different friend managed to get a RAM stick in an AGP graphics card slot (What the **** they are different shapes), this did cause damage to the system.
I take it power LED, power buttons, HDD activity LED, reset button have already been wired in so none of those m/b pins could be to do with that.
