First of all, you should probably not be using the beta version. I mean it's OK if you do, but you will have problems, as even the most experienced users are having. Version 6.0 is stable, and much nicer to learn on than one that is actully not even a beta yet.
Secondly, I don't know why you are getting two voices. If you were not using the beta, the problem might be easier to find, but I still wouldn't know since I have never run into it. Perhaps you could post as much information about your system and the voices you have installed as possible and then maybe someone can help.
Lastly, the dialog for the appointment book is in the Appointments.uhp file if I remember correctly. Some of the sentences may be concantated, so only edit this if you know a little about scripting.