MS Voices (Microsoft voices) are free voices supplied by Microsoft for use with Windows. They are known fully as Microsoft text-to-speech voices.
Alter/ego redirects the API into SAPI for Alter/ego, allowing the voice to show up within the software providing they are installed. These voices will register for Alter/ego though are limited by their original capabilities, meaning a talker will remain a Talker.
MS Sam, Mike, Mary, Hazel, David, Anna and Zira can all be registered. Note that pre-Windows 8 vocals from the series can be harder to install in the first place, before they can even be considered for Alter/ego.
Placeholder avatars are in the process of being made by volunteers.
This feature is not yet implemented, but is due in a future update.