www.ottrproject.org
Open-source Tools for Training Resources (OTTR) helps you write content once and publish it to 3 different platforms: Bookdown, Leanpub, and Coursera!

loqui.fredhutch.org
An app to create videos from Google Slides using ARI. The audio component is derived from the slide notes, while the slide images are used to create the visual elements of the video.

cran.r-project.org/web/packages/text2speech/index.html
An R package that unifies different text to speech engines, such as Google, Microsoft, and Amazon.

https://cran.r-project.org/web/packages/ari/index.html 
ARI (Automated R Instructor) is an R package that automates the creation of videos from html, markdown and other sources.

https://cran.r-project.org/web/packages/conrad/index.html  
Conrad is an R package that acts as a client for the Microsoft's 'Cognitive Services Text to Speech REST' API to convert text into synthesized speech using a variety of voices.