how to download audio form youtubedl class object
-x, --extract-audio Convert video files to audio-only files
(requires ffmpeg/avconv and
ffprobe/avprobe)
--audio-format FORMAT Specify audio format: "best", "aac",
"flac", "mp3", "m4a", "opus", "vorbis",
or "wav"; "best" by default; No effect
without -x
--audio-quality QUALITY Specify ffmpeg/avconv audio quality,
insert a value between 0 (better) and 9
(worse) for VBR or a specific bitrate
like 128K (default 5)
--recode-video FORMAT Encode the video to another format if
necessary (currently supported:
mp4|flv|ogg|webm|mkv|avi)
--postprocessor-args ARGS Give these arguments to the
postprocessor
-k, --keep-video Keep the video file on disk after the
post-processing; the video is erased by
default
--no-post-overwrites Do not overwrite post-processed files;
the post-processed files are
overwritten by default
--embed-subs Embed subtitles in the video (only for
mp4, webm and mkv videos)
--embed-thumbnail Embed thumbnail in the audio as cover
art
--add-metadata Write metadata to the video file
--metadata-from-title FORMAT Parse additional metadata like song
title / artist from the video title.
The format syntax is the same as
--output. Regular expression with named
capture groups may also be used. The
parsed parameters replace existing
values. Example: --metadata-from-title
"%(artist)s - %(title)s" matches a
title like "Coldplay - Paradise".
Example (regex): --metadata-from-title
"(?P<artist>.+?) - (?P<title>.+)"
--xattrs Write metadata to the video file's
xattrs (using dublin core and xdg
standards)
--fixup POLICY Automatically correct known faults of
the file. One of never (do nothing),
warn (only emit a warning),
detect_or_warn (the default; fix file
if we can, warn otherwise)
--prefer-avconv Prefer avconv over ffmpeg for running
the postprocessors
--prefer-ffmpeg Prefer ffmpeg over avconv for running
the postprocessors (default)
--ffmpeg-location PATH Location of the ffmpeg/avconv binary;
either the path to the binary or its
containing directory.
--exec CMD Execute a command on the file after
downloading and post-processing,
similar to find's -exec syntax.
Example: --exec 'adb push {}
/sdcard/Music/ && rm {}'
--convert-subs FORMAT Convert the subtitles to other format
(currently supported: srt|ass|vtt|lrc)