Unfortunately it was basically impossible to understand what it was saying in a moving vehicle.

Do you think it was the quality of the voice itself that was sub-par? The volume? Or something else? All these can be worked out... I haven't tried the new Flite out yet, but according to the docs, it can use higher quality voices, but it looks like a massive project to get voices converted from FestVox to Flite.

If it's just a problem with the volume, that's easy to work with. The speed and frequency of the voices are also easy to change. What exactly did you think the problem was with the ttsd/Flite solution?