hhhhmmmmm.... i have worked a little with .ogg and .wav and mp3's, but not alot, you probably have more experience than i, but wanted to ask if there was a stereo, or a per channel option in the .wav? Voice may only be one channel, but this is only a guess on my part. what if you mix in low level chatter in the background for the second channel?
My guess at this point is it is the .wav file you are using. Try another voice or see if there are differences in the original .ogg files that play inside Sins.
I can fire up the ogg converter and see if i can see anything from files i have here, but dont know if i can find anything that i will be able to understand.
Sorry to be vague, but wanted to try to help, even though my experience is limited.
-Teal