Hi Daniel,
I too have just purchased the same software as yourself and I find the same problem with the voice only being output to the left channel.
To date this is one of the few problems that I have had with my mac.
Having spent days and days reviewing all the available mac voice products I came too the conclusion that the AT&T voices are the most natural sounding so I think that you made the best choice in choosing them. Its unfortunate that they seem to contain these minor glitches.
Are these types of problem standard in the computer industry.Unfortunately the answer to that is Yes.Count yourself lucky that you are using a mac rather than a pc or you would simply take it for granted that there would be something wrong with the software.

Best Regards

Clive