This question already has an answer here:
- Natural Sounding Text to Speech? 14 answers
This question was asked here, four years ago: Natural Sounding Text to Speech?
However, text-to-speech is something that's still improving and evolving fast, and all of the answers yield software that sounds tinny or unnatural. I know that no TTS will be perfect, but I'm positive there must be better solutions now in 2015.
The size of the package doesn't matter; it doesn't have to be lightweight, it can be bloated for all I care; I only care about the end result. It only has to be free and available for Ubuntu.