AT&T provides a free reader at this URL:
http://www.naturalreaders.com/download.phpIt is possible that this application will grant you access to it's control panel. It works that way with Ivona Voices, but I have not tried to adjust the speed of Natural Voices.
EDIT:
I was in error about the above link being sponsored by AT&T.
On further investigation, it appears that AT&T provides the speech technology but does not make consumer level products to directly affect the technology.
It seems that speed support is provided in the SDK but it is up to the developer utilizing the speech technology to provide whatever interactions they feel are needed. So if the capability was not available in your Zabaware Natural Voices product then it can't be done.
However if you have obtained the voices through a different application suite, then you may be able to adjust read speed system wide through any software solutions that they provide. One product I can think of that is configurable and uses AT&T voices is NExtUP's TextAloud
http://www.nextup.com/TextAloud/(I don't guarantee that it retains speed settings across all TTS applications)
Here is a link to a screenshot showing the voice controls:
http://www.nextup.com/imgnew/tascr.jpg