I was sure I had already replied to this...
SAPI is a Microsoft technology. The Linux Kindle doesn't work with SAPI voices. The only thing that looks like it would make SAPI work on any kind of Linux is this:
http://code.google.com/p/open-sapi/
But it requires WINE, which I don't think we will ever get on the Kindle. Also, if SAPI voices have any compiled code in them, then they won't ever work on the Kindle. It uses a different processor than a desktop computer.