This is one of my favorite topics, probably because I have been playing with synthesized speech on computers since the TRS-80 Voice Synthesizer in '79.
Go to:
Edit > Speech > Settings
What Engine is Scrivener set to? I’ve been using winrt because the voices sound better than sapi voices.
Windows 10 comes with David and Zira.
If you go into Windows settings and search for speech settings, you can download voices for your computer. I’ve downloaded George and Hazel. They sound more natural, in my opinion.
Make sure you have a language/country selected in Scrivener that has voices installed for it.
My computer isn’t set up to use the mock engine. So those voices have no audio. Mock could just be a mock-up of some kind for testing (on my computer, at least).