static DownloadedMedia downloadAudio(WithURL<?> url) {
  if (url == null) null;
  ret downloadAudio(url.url());
}
  
static DownloadedMedia downloadAudio(S url) {
  if (empty(url)) null;
  File dir = downloadedAudioDir();
  S extension = ".opus";
  
  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);
  S options = "-U --no-playlist --no-mtime --write-description --write-info-json"
    + " --write-annotations --write-thumbnail --print-json --restrict-filenames";
  options += " -x"; // audio only
  options += " --audio-format opus"; // demand opus
  
  S 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);
}