Realtek ALC889A onboard not mixing down centre to 4 or 2 speakers properly

I've got a Gigabyte MA780G-UD3H, with onboard ALC889A audio. I'm running XP SP3.

The problem is obvious in some games, namely Assassin's Creed and Prototype, where some of the characters speech is inaudible when I'm using 2 or 4 speakers. If I set it to 6 speakers in the Realtek control panel and plug in a small battery powered speaker as a centre, it's fine.

I've updated to the latest 2.27 driver from 2.25 but it hasn't made any difference. I also tried reducing the accleration to basic. Does anyone know if this is a known bug in the driver that's being worked on or a known bug in the games?

The speakers are setup fine. Obviously when I'm in 2 channel mode I can only test the two speakers but they work fine. Just seems that Directsound isn't mixing down properly for some reason.

I can't see any "large/small" speaker setting in either the Realtek Control Panel or the XP one. With "2 speakers" selected in the Realtek CP, the XP CP is set to "Desktop stereo speakers", "4 speakers" changes it to "Quadrophonic" and "6 speakers" changes it to "5.1 surround sound system".

In the Realtek CP, under the Sound Effect tab Environment is set to None and the equalizer is flat, under the Microphone tab all the features are disabled and none of the features along the bottom (Dolby Headphone, Pro Logic II, Virtual Speaker, Digital Live) are enabled.

I don't have those options under XP.

I go Control Panel - Sounds and Audio Devices which opens a window with a "Speaker settings" section at the bottom containing an Advanced button. Clicking that brings up the windows where I can change the speakers from "Desktop stereo" to "Quadrophonic" or "5.1 surround" but there's no small/large speakers setting or any Test function.

Note that changing this setting doesn't cause the Realtek CP speaker setting to change, whereas changing the Realtek CP setting does change the XP CP setting.