Well, I just registered to thank the OP (and subsequent posters) for this nice trick, and to confirm that it still works with the latest Realtek chips on LGA1155-based mobos with the latest realtek drivers
(2.56 at the time of writing).
As it has been mentionned before, the location of the key depends on your system but with a few searches I found it easily. For me (Windows 7 Pro 64b, Realtek drivers 2.56) it was there :
From there, I just had to change the value of Pin02 from 05 00 00 00 to 04 00 00 00 to replace rear speaker output by a second front speaker output so that i could plug speakers and headphones simultaneously.
Works like a charm! Thanks again.