You have STAC9708 and the secondary I2S codec for the rear channels.
I am not sure if register 36, or register 38, is the rear channels on your card (it is not always used the way it is listed in the datasheet). If it is register 38, than the above command is unmuting the center/sub channels instead of the rear channel, and that would cause problems with your digital output.
Try playing a song, using analog out only (DOO mode off), and type the following in kX Console.
sac97 36 8080
See if that mutes the rear channels or the center/sub channels.