After playing around with the agent trying to get the text-to-speech to work, I started searching this forum, and found a hint in an old post that you should set the langid on the agent to get it to work with the TTS packs from Microsoft.
/gopts -n agent 0x0409

Was wondering, wouldn't this be worth mentioning either in the help file or on the agent page: http://www.mirc.co.uk/agents.html
or perhaps place a "select langid" dropdown in the mirc settings for agent ?