I have a diablotek legend mid atx case and when i disconnected the hd audio cable from the motherboard i popped one end of a wire off that kind of loops back into the connector, i checked my motherboard manual and the end that is still connected is labeled as ground and every other slot is filled except for the one that is closed so basically I need to know where the other end of that wire goes back in or if ill just have to screw the wire to the case to ground it.
i believe that would be the shared ground between the headphones or mic ports.
you will need to connect it and see which one it is. if it is a port you won't use then its not a problem.
edit: if it causing a problem find out where it should go and try using something small like a paper clip to "re-insert" it back in the plug. it only need to make a connection, then secure it with electrician's tape so it doesn't come out again.