code: func (r *Recording) URL(extension string) string {
  	if !strings.HasPrefix(extension, ".") {
  		extension = "." + extension
  	}
  	return strings.Join([]string{BaseURL, r.APIVersion, "Accounts", r.AccountSid, recordingsPathPart, r.Sid + extension}, "/")
  }
docs: // URL returns the URL that can be used to play this recording, based on the
  // extension. No error is returned if you provide an invalid extension. As of
  // October 2016, the valid values are ".wav" and ".mp3".