For anyone still using XP who is interested, Code
Factory states that MSAPI5 Eloquence works in Windows Vista, 7, 8, and 10.
I tried it on my XP Service Pack 3 machine today and it runs properly.
Many years ago, I tried it on another XP machine I no longer have running
Service Pack 2. It ran in that as well so it will probably run in XP in
general.
Also, since an MSAPI5 synthesizer should run in any
program that supports MSAPI5 speech, it should run in the beta versions of NVDA
as well as in the earlier versions. So it should make no difference if you
are going to use the new versions as they come out or continue to use an old
one.