Otherwise a bit of programming skill, it should be easy to do it - I would be surprise if no one hasn't yet. Unfortunately I don't specifically know a dedicated application for this, but for Windows this might contain what you need. 0 / 5000 Current Limit: 6000 characters per week.

Convert text to speech free online and download it as Mp3 in natural voices. You can always redirect audio output into audio capture device (you can do that on windows with the audio loopback device, and most OSes support a way of doing this), in which case all the audio output of your computer, such as YouTube audio, can be captured by the said APIs. Visit to use online text to speech converter today Home Support Buy More Characters Member Zone Free TTS: Text to Speech Mp3 Free Online.

Otherwise, albeit slower, you can use Google Cloud Speech API, or IBM Watson, and Amazon's Alexa Voice Server to do the same with internet connection. On Windows there's the Speech Recognition API, which is very easy to use, as in a few function calls, and you're all set.

Mp3 speech to text software