There are two other things you can try.
1. Open the Speech control panel (assuming you installed it) and check if there are any TTS engines installed. If not, you will see an empty window. Otherwise click on one of them (e.g. TruVoice) and then on Pronounciation. Type some words and see if they are actually spoken. If that works, you can be sure that the problem is not with SAPI 4 or with the TTS engine.
2. Your Windows version could be non-US English. For example, if you have a Dutch Windows version and only one TTS engine which happens to be US English, your agent characters will refuse to talk because they can only speak Dutch.
Try the following commands in your Perform section:
/gload mirc
/gopts -n mirc 0x0409
The first line loads whatever character is selected in File/Options/Sounds/Agents, the second line sets the language to US English. Now all you have to do is connect to a server and see what happens. Good luck.
