/*

Written by Amit Agarwal
email: amit@labnol.org
web: https://digitalinspiration.com
twitter: @labnol

*/

function convertAudioToText(flacFile, languageCode) {
  var file = DriveApp.getFilesByName(flacFile).next();
  var bytes = file.getBlob().getBytes();

  var payload = {
    config: {
      encoding: 'LINEAR16',
      sampleRate: 16000,
      languageCode: languageCode || 'en-US',
    },
    audio: {
      // You may also upload the audio file to Google
      // Cloud Storage and pass the object URL here
      content: Utilities.base64Encode(bytes),
    },
  };

  // Replace XYZ with your Cloud Speech API key
  var response = UrlFetchApp.fetch('https://speech.googleapis.com/v1/speech:recognize?key=XYZ', {
    method: 'POST',
    contentType: 'application/json',
    payload: JSON.stringify(payload),
    muteHttpExceptions: true,
  });

  Logger.log(response.getContentText());
}

curl --silent --insecure --header "Content-Type: application/json"
"https://speech.googleapis.com/v1/speech:recognize?key=XYZ"
--data @payload.json

// Content of payload.json
  {
    "config": {
        "encoding":"FLAC",
        "sampleRate": 16000,
        "languageCode": "en-US"

    },
    "audio": {
        "uri":"gs://ctrlq.org/audio.flac"
    }
  }