Let me preface this with the fact that I've been working with the voice/speech functions of mh for less than a week now, and with mh in general for only several months (and those not consecutively)! With that in mind, here's what I've found related to your question:
There are quite a few sound functions in the module ./code/common/mh_sound.pl including ways to change the speech patterns via voice commands. All voice commands can be displayed by the voice command of voice commands: "list voice commands".
The settings you are wanting to work with can be adusted within Windows through Control Panel | Speech | Text to Speech . Assuming that you downloaded MS SDK 4.0, you should have an option for a few voices : Mary, Sam, etc. There's a slide bar in this same window for adjusting the speech speed.
Also, as far as the voice function just quitting on you, I found a perl module ./code/bruce/voice_ms.pl that appears to address a microsoft bug for this function.