fix: review comments

lib/synth-audio.js

@@ -52,7 +52,7 @@ const TMP_FOLDER = '/tmp';
  * @returns object containing filepath to an mp3 file in the /tmp folder containing
  * the synthesized audio, and a variable indicating whether it was served from cache
  */
-async function synthAudio(client, logger, stats, {
+async function synthAudio(client, logger, stats, { account_sid,
   vendor, language, voice, gender, text, engine, salt, model, credentials, deploymentId, disableTtsCache
 }) {
   let audioBuffer;
@@ -95,7 +95,6 @@ async function synthAudio(client, logger, stats, {
   } else  if (vendor.startsWith('custom')) {
     assert.ok(credentials.custom_tts_url, `synthAudio requires custom_tts_url in credentials when ${vendor} is used`);
   }
-  const account_sid = credentials.account_sid;
   const key = makeSynthKey({
     account_sid,
     vendor,