I found an article on MSDN describing how the flight simulator ATC (and pilot I’m guessing) voices are created. Believe it or not, they’re not computer generated but rather a splicing of *.wav files in the right places. ATC voices have been in flight sim since 2002 and they’ve really added to the realism. It’s quite interesting that all was done through some fancy editing.
I happened upon this article while looking into an SDK for flight sim. Unfortunately I have yet to find a free one, preferably from Microsoft itself. If you happen to know of a free flight simulator SDK, please let me know!