On Windows, I use Balabolka to do this.

There's also a Python package on pypi called pyttsx3, which I used to write a very simple command line program to say whatever is on my clipboard.

http://www.cross-plus-a.com/balabolka.htm

https://pypi.org/project/pyttsx3/

I just use the one built into Mac OSX actually. But Hearling.com is also a quick way to download audio files.