你做得很对，但是使用的是旧方法，请将你的代码更新为下面提到的代码，同时检查最新的youtube_explode_dart包的文档以更好地理解。

Future<String> getDownloadUrl(String videoId) async {
var youtube = YoutubeExplode();
var video = await youtube.videos.get(videoId);

// Get the stream manifest for the video
var streamManifest = await youtube.videos.streamsClient.getManifest(videoId);

// Get the audio-only streams from the manifest
var audioOnlyStreams = streamManifest.audioOnly;

// Get the highest quality audio-only stream
var audioStream = audioOnlyStreams.withHighestBitrate();

// Return the URL of the audio stream
return audioStream.url.toString();

}

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正如在官方文档中所提到的，audioOnly属性仅用于此目的。>最高比特率的仅音频流。您可以再次查看官方文档以获取更多信息。https://pub.dev/packages/youtube_explode_dart

// Get highest quality muxed stream
var streamInfo = streamManifest.muxed.withHigestVideoQuality();

// ...or highest bitrate audio-only stream
var streamInfo = streamManifest.audioOnly.withHigestBitrate()

// ...or highest quality MP4 video-only stream
var streamInfo.videoOnly.where((e) => e.container == Container)

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