That's always how I imagined Fran sounding. I imagine it would actually be a bit of a challenge for the seiyuu to convey the sharp contrasts in Fran's personality. Are Veronica and Gavril in the audio drama too? I'm kind of interested in hearing it now.
I think Reki's seiyuu, Junko Noda, did a fine job, although I'm having a hard time thinking of specifics. Her voice is kind of low-pitched, especially for a Japanese woman, and it has a slight husky quality. I'm going to pay closer attention the next time I rewatch the series.
Although this thread made me curious to watch the dub, which I haven't seen in what must be almost ten years now. And was it bad, oh my, it was awful. I remembered thinking her english VA was okay back then, but now I could hardly distinguish her voice from Rakka's when they were both on screen.