The best TTS voices available for the Mac just got updated, iVox from Holland’s Assistiveware is now running at v1.2. There's no patch or update route so you are going to have to download a whole new version and reinstall and at over 700mb it’s a bit of a beast. But in v1.2 you get the ability to add abbreviations or change the pronunciation of words. It works pretty well I personally prefer the US Heather voice in iVox but I don't want to hear words like aluminium pronounced in her native tongue so I can go into the Pronounciation Editor and have aluminium edited so that it sounds exactly like an English TTS would.
Also included in version 1.2 are a selection of new languages Eliska and Sabrine for Czech, Ania for Polish, Snorri for Icelandic, Sanna for Finnish, Elin for Swedish, and Baris and Ceren for Turkish. The Czech, Polish, Icelandic and Turkish voices require Mac OS X 10.5. The original voices require 10.3.9 or later.
You can download the full working version of iVox on a 30 day trial
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Listen to Samples
Download v1.2