The Speech control panel that comes with PlainTalk 1.4.x has an option to adjust the rate of a particular voice. Macintosh Guide describes this function as follows:
"If you have a favorite computer voice, you can use the Voice option of the Speech control panel to specify it as the default voice. You can also indicate how fast the default voice will speak in most programs.
The default voice reads text aloud in most programs with text-to-speech capabilities, unless you specify otherwise. (Some programs have their own default voices and do not follow this setting.)"
When you set the rate in the "Voice" portion of the Speech control panel, you are setting the rate of the "Default voice." Only programs which offer a "Default voice" choice in the Voices sub-menu will be able to use this rate. SimpleText does not currently offer a Default Voice selection in the Voices sub-menu and therefore will not play voices at different rates, no matter what is selected in the Speech control panel.
This behavior may change in future versions of SimpleText or PlainTalk software.