static DownloadedMedia DownloadAudio(WithURL<?> url) {
  if (url == null) null;
  ret downloadAudio(url.url());
}
  
sclass DownloadAudio {
  S url;
  File dir = downloadedAudioDir();
  new LS options;
  S extension = ".opus";
  int maxFileSizeInMB = 20;
  
  S cmd;
  
  *(S *url) {
    options.add("-U --no-playlist --no-mtime --write-description --write-info-json");
    options.add("--write-annotations --write-thumbnail --print-json --restrict-filenames";
    options.add("-x"); // audio only
    options.add("--audio-format opus"); // demand opus
  }
  
  DownloadedMedia get() {
    if (empty(url)) null;
    
    options.add("--max-file-size " + maxFileSizeInMB + "m");
    
    S fn = md5FileName(url);
    File infoFile = newFile(dir, fn + ".info");
    S json = loadTextFile(infoFile);
    if (nempty(json)) {
      DownloadedMedia dm = new(null, json, extension);
      if (fileExists(dm!)) ret dm;
    }
    
    saveTextFile(newFile(dir, fn + ".url"), url);
    
    cmd = pqO(installYouTubeDL())
      + " " + options
      + " -o " + pqO(newFile(dir, youTubeDLDefaultFileNameTemplate()))
      + " " + pqO(url);
    S output = backtick_verbose(cmd);
    saveTextFile(infoFile, output);
    ret new DownloadedMedia(cmd, output, extension);
  }
}