Training an acoustic model for a speech-to-text engine
This paper has details on how to prepare Audio data (and merge it with Language Models) for speech-to-text :



http://slazebni.cs.illinois.edu/spring17/lec26_audio.pdf

Slide 16 has very high level description of feature engineering. 



Following papers provide more detailed analysis of audio processing for speech to text :

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.701.6802&amp;rep=rep1&amp;type=pdf
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.689.4627&amp;rep=rep1&amp;type=pdf
https://www.ijariit.com/manuscripts/v4i3/V4I3-1646.pdf