The text-to-speech synthesizer is not actually built-in to mIRC, but mIRC can command and control a speech component that has been installed on your computer. To read more about this feature, refer to this link about Microsoft Agent
as well as the command /help Speech
The scripting for this command isn't that difficult, but scripting in general becomes more challenging the more intricate your needs or desires to make it work 'just so.'
I can lend you a hand if you give me a bit more information about your specific needs and how you use mIRC. Are you visually impaired, what are your current challenges getting around in mIRC, what areas is voice synthesis currently working well, not working well, and what considerations would make it easiest for you to use mIRC otherwise?