If you're looking to improve the voices you currently have, hopefully this guide can help!
This guide from Microsoft contains lot of info like:
- where to find the voices currently installed
- how to install more voices from Microsoft
- third party vendors where you can purchase voices
- open source projects with free voices
As helpfully pointed out by user jeofo in this issue, Mac users can access enhanced and premium versions of many of the voices available. See this guide for installing more voices on Mac.
On Linux machines, ZoTTS relies on speech dispatcher to run TTS so familiarising yourself with that will help.
You can find their complete documentation here. Documentation about supported modules is here and how to configure them is here.
Speech dispatcher is fairly complex, so I'd recommend taking your time and asking them if you run into trouble (I would help if I could but I'm not a linux native and the original devs will be much better placed to assist you!)