This seems to work for both TTS and TTSMS plugins
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.1.3.6-->
<command id="221" name="spit it out" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>TTSMS.Speak</cmdType>
<params>
<param><rate absspeed="8">I am talking really fast</rate></param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>TTSMS.Speak</cmdType>
<params>
<param><rate absspeed="-8">I am talking really slow</rate></param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
</command>
Personally I find that you can't really mess with the speed too much, or the voices will sound even less real.