Ouch, I didn't know that. I'm on a 7.1 setup even. I'm using the Soundblaster MB3 software, so I can expand sound sources to surround, but still.
I have an on-board Realtec for the sound, it has also a 'speaker-fill' option, but it expands only stereo sound for the surround speakers, and leaves the 'center-only source' untouched .
the best I could do is to use in VA the 'speech' and 'say' EDDI plugin commands (they use EDDI's speech engine) instead of the built in 'say something with text-to-speech' command. A bit more work at scripting, but speech sounds the same way all the time.
I'm happy with this already, all other sound improvements in EDDI are "cherries on the top of the cream" .