This tutorial will walk through using Google Cloud Speech API to transcribe a large audio file.
All code and sample files can be found in speech-to-text GitHub repo.
— Google Cloud (@googlecloud) January 11, 2018
This approach works, but I found that result will vary greatly based on the quality of input.
Transcribing a Reading by My Wife
I asked my wife to read something out loud as if she was dictating to Siri for about 1.5 minutes. She is a native English speaker and we recorded using a microphone on iPhone 6s.