There's a software i am now using to simulate voice in my missions.
Its called Talk It.
And the voices are natural (in my opinion)
You can check the voice sample here :
http://www.text2speech.com/sv_exam1.wavIf you like it,just send me the lines.
And don't forget to mention the type of human that's saying the words
(Standard Man, High Pitched Man/Strong Man, Radio chatter,Old Woman,Childish Man,Nerd etc)
Maybe send me about three lines and i'll send your the results,if you are satisfied you can go one with it.
Just willing to help someone, that's all........
Regards,
Haroon1992