I remember for MP!3, there was a plugin that would read the person your message through MS Sam.
So you send your text with a command and you and the other person would hear it through MS Sam. But only the text that is sent through the command it relayed through MS Sam.
I know there is the +Speech script, but it enables you to hear other peoples messages. The SpeakIt would only read the message if the persons sending it put the command in.
Hopefully that makes sense