Where did my rear speakers go?
With the current Realtek HDA drivers and HD Audio Manager, the rear speakers don't work with 6 speakers unless 8-channel mode is selected. You may be surprised to know that 5.1 channel doesn't have rear speakers but surround speakers, which are actually placed on either side. Your 6 speakers don't include rear speakers.
Don't believe me? Check the set-up and placement guides from Dolby and THX:
Realtek HD Audio Manager and the missing Speaker Fill option
First go to Playback devices
Then select the Realtek speakers and choose Properties
On the Dolby tab, check Dolby Prologic IIx and you're done.