IMspeak Adds Speech to Instant Messages
Description from topshareware.com:
IM Speak! is a tool that adds speech and personality to incoming messages from your favorite Instant Message program. By using the program, you can assign 'voices' to each one of your buddies allowing them to speak to you in any voice you choose along with many other features.
Click on Topshareware.com's website http://www.topshareware.com/IMSpeak!-download-2781.htm for the free download (if you like it, it's kind of pricey--$24 at last check...).