I was disappointed with Logos 5, when I bought it and found the text to speach was the exact same technology as version 4. The robodic voice is obnoxious at best. What are the chances Logos will ever upgrade this to a usable version...?
Logos uses the default voice in Windows. It's out of their control.
You can buy third party software to get different voices. I use a text-to-speech voice from IVONA, which I got packaged in a program called FlameReader. There are many other vendors and voices out there as well.
I too have used IVONA for a while. You can go to their web site and there are quite a few choices in voices, very high quality, different accents, male/female, etc. It's quite pleasant to listen to!