如何使用音频下载视频以使用youtube-dl嵌入式脚本？

Question

我使用此脚本下载视频。视频已下载，但没有音频。

./ download.py

#!/usr/bin/env python

# encoding: utf-8
from __future__ import unicode_literals
import os, sys, youtube_dl

def download(url):
     ydl_options = {
         "outtmpl":           "%(id)s.%(ext)s",
         "writethumbnail":    True,        
         "addmetadata":       True,
         "format":            "bestvideo/best[height<=720]+bestaudio/best[height<=720]",
         "continuedl":        True,
         "quiet" :            True,
         "restrictfilenames": False,
         "ignoreerrors":      True
     }
     with youtube_dl.YoutubeDL(ydl_options) as ydl:
        try:
            ydl.download([url])
        except (youtube_dl.utils.DownloadError,youtube_dl.utils.ContentTooShortError,youtube_dl.utils.ExtractorError) as e:
            print(e)

def main():
    download(str(sys.argv[1]))    

if __name__ == "__main__":
    main()

如何下​​载分辨率小于或等于720p并带有音频的视频。