I've written a series of scripts to make mirc speak better than the default. What you need is a text to speech engine. I would suggest using something other than microsoft sam, as that voice can really get on your nerves. Look at
www.nextup.com - they have a good deal of voices. I am blind, so this topic is very near and dear to muh lil heart. If you have further questions, I am Asterisk_ on undernet.