There's a link here for Mary/Mike voices install:
http://www.jaws-users.com/programs/
There should be other links online. They're not
official downloads. Microsoft has been stingy with
the SAPI voices. They typically come only with this
or that software. But some 3rd-party entities have
been generous enough to build installer for them.
You can also use the MS SAPI SDK link, but it's
more work. Download Sp5TTIntXP.exe instead of
the SDK. It contains the voices in a merge module
for Windows Installer. Unpack the MSM file with
any of the unpackers from here:
http://www.jsware.net/jsware/msicode.php5
Copy the voice files to the speech folder and add
the Registry entries that the unpacker log lists.
Any voice can be added that way, but it takes
a little work. In addition to copying over the .sdf
and .spd files, a handful of Registry entries have
to be added under
HKLM\SOFTWARE\Microsoft\Speech\Voices\Tokens\.