What are you using for text-to-speech? mIRC is a great program, but
it may not be handled properly by all t-t-s programs. Download it and
try it first. (Or have you already done that, and are just looking
for a free alternative?)
If you mean you want notification when someone types to a channel
you're not watching, look at mIRC scripting.
Generate a .wav (or .mp3) file for each channel you want to be
notified about and have anything said in the channel play that sound
file. So, if someone typed a line in #freeware, you could hear
"freeware" or "undernet freeware", or whatever you wanted. (It's
usually enough for me to hear the "ding.wav" that mIRC normally plays
on a channel not in the foreground.