We would like to do voices at some point. What we've seen in other games is like you guys said where the voices tend to get stale after a bit.
Another thing that really turns us off at least is when the things the girl says don't match what's going on. No mention of any specific games but that happens quite a bit in one of them. Plus, we REALLY REALLY care about SEXY sound, and in some of the games it just sounds like they recorded fake sex in a sound room. Our sound is recorded from a live couple actually having real sex while recording, so we would want the same thing with voices.
We've tried capturing voices just in a mic without actual sex going on, and there's just something subtle missing. We're all about the sexy subtlety so if we do voices we want them to be super ultra hot and not just some random sayings that don't match what's going on on screen. It is something we plan to do at some point, but we want to get it right before we do.