I have purchased Power Translator Premium 15 directly from LEC website (Language Engineering Company) trusting their documentation regarding the Text-to-Speech feature, since I was esp. interested in listening to Russian pronunciation.
Unfortunately, this function was deactivated and I was told to download the Microsoft Language Pack to fix this issue. However, Windows Vista Home Premium users cannot download this pack from Microsoft which is restricted to Vista Ultimate or Vista Enterprise users only. Such limitations seem also to apply to XP users.