Not sure what this could be yet, I'll need some additional information.
First, can you describe exactly what is happening when you're not able to change voices? Does the voice change in the Outlook dropdown list, but TextAloud doesn't actually switch voices? Is the voice dropdown list disabled? Are you seeing any error messages?
Second, it may help if you duplicate the problem in Outlook, then go to the TextAloud main window and click Help -> Report A Problem. I may be able to pick up some things from the debug information included in the problem report.