Because development of text to speech voices has traditionally been driven by call centers, companies have generally gone for the most standard, non-accented speakers they can find. I suppose it makes sense, if you are doing a nationwide call-in center, you don’t want your automated voices sounding like they are from Boston or Georgia, you want just a plain vanilla American. And that is what you get with most US English voices. We’ve been able to get some variety with great British voices like Audrey and Australian Karen. AT&T and Nuance have added Indian accented voices, and the first South African voice is on the way, but there still aren’t a lot of choices.
I’m not sure traditional businesses are driving demand for more variety, but online uses and general consumers are always looking for more personality in voices. I’ve heard requests for everything from southern red-neck to inner-city black kid, and about everything in between. Nothing to offer in those areas yet, although it will come, but it did prod me into this interesting experiment. I took all of the non-english speaking voices we offer and gave them a shot at a paragraph of English text. In some cases the results were terrible, in some cases tolerable, a few cases funny, and with a few of them, they were fantastic. Listen to all the results at the NextUp.com Accents Page.