MLSpeak is a Text to Speech plugin. It requires a SAPI 5 compliant "voice" installed on the MLServer3 Windows PC. Windows comes with a default SAPI5 voice. There are commercial SAPI5 compliant voice engines available for purchase by a few vendors that are much higher quality voice reproduction.
Here is one source: Homeseer
You would adjust speaking rate within the SAPI5 voice driver (usually in Windows Control Panel).
Supported MLServeCmd Syntax
MLServeCMD.MLSpeak|Any text you want
MLServeCMD.MLSpeak|Voice~Any text you want
MLServeCMD.MLSpeak|Voice~Any text you want~rate
Voice = Anna, Michael, Michelle or whatever voices are installed on your MLServer PC.
Rate = -10 to 10