For those who are running Logos 4 on Windows 7 and do not like the default Microsoft Anna voice, I finally found a way to select and use other voices (e.g. Paul and Kate from Neospeech / Nextup):
Run C:\Windows\sysWOW64\speech\SpeechUX\SAPI.cpl
This brings up a window that looks just like the Text to Speech window in Control Panel, except that your third-party vendor's voices will also show up there and can be selected as Windows 7's default voice. You will have to restart Logos 4 for the new voice to take effect.
Credit: http://social.technet.microsoft.com/Forums/en/w7itproinstall/thread/c7409091-087e-48f2-8178-bcd918799054
Can someone else test to see if it works for you also? If so, we should add this to the Wiki.
[Edit: I have 64-bit Windows 7. Not sure if this is also a problem for 32-bit.]
Peter