its the ALC888, ive read all realtek HD audio above 880 still has bad drivers for source/valve games.
There are older version of the Realtek drivers that do work properly, installing an older version will give you 5.1 sound. This is only for the AC97 series (AC97 up to ALC850).
The latest HD_Audio Chip (the ALC 880D) does not have a fixed driver available yet, but I just got official word from Realtek that they know about the problem and are working on a solution.
This problem is (and has always been) a Realtek driver problem, not an HL2/Valve problem.
For now (for the 880D) there is no solution except to use stereo mode."