code: private String downloadVoice(String msgId) {
          return this.downloadFile(
                  new DownLoad(ApiURL.VOICE, bot.session().getUrl(), msgId, bot.session().getSKey())
                          .msgId(msgId).saveByDay()
          );
      }
docs: 下载音频到本地
  
  @param msgId 音频消息id
  @return 返回音频本地路径