Не-а. Если формат указан явно, то качается видео. И быстро:
$ youtube-dl --version
2017.12.23
$ youtube-dl -v -f webm -x --audio-format mp3 https://www.youtube.com/watch?v=bv3-E92qvxY&list=PLGJT6m6pbQ2BYzrbAdBsQ1hQ4_JQ9giiB
[1] 1770
$ [debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '-f', 'webm', '-x', '--audio-format', 'mp3', 'https://www.youtube.com/watch?v=bv3-E92qvxY']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.12.23
[debug] Python version 3.5.4 - Linux-4.4.97-std-pae-alt0.M80P.1-i686-with-fedora-ALT-Linux
[debug] exe versions: avconv 11.6-alt1, avprobe 11.6-alt1
[debug] Proxy map: {}
[youtube] bv3-E92qvxY: Downloading webpage
[youtube] bv3-E92qvxY: Downloading video info webpage
[youtube] bv3-E92qvxY: Extracting video information
[debug] Invoking downloader on 'https://r1---sn-cxauxaxjvh-hn9ez.googlevideo.com/videoplayback?key=yt6&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cexpire&mm=31%2C26&mn=sn-cxauxaxjvh-hn9ez%2Csn-4g5e6nle&mt=1533738970&mv=m&pl=19&ei=RgBrW5K0Co2Y1gLnxJ2gBA&ms=au%2Conr&signature=28093ED81C4F81E80531E106E29539366D1517E8.CD436A39C48CCECDD4779FC59B08FE1B3DD9831D&clen=71609219&ip=37.212.23.117&itag=43&lmt=1492862896948884&requiressl=yes&id=o-AJKRy2cPCCmMadrGdHp3jg4mzjpdHV0ZjOZ9TvcNDcgO&source=youtube&dur=0.000&mime=video%2Fwebm&initcwndbps=622500&ipbits=0&ratebypass=yes&expire=1533760678&fvip=1&beids=%5B9466592%5D&gir=yes&c=WEB'
[download] Destination: РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm
[download] 100% of 68.29MiB in 00:22
[debug] avconv command line: avprobe -show_streams 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm'
[ffmpeg] Destination: РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.mp3
[debug] ffmpeg command line: avconv -y -i 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm' -vn -acodec libmp3lame -q:a 5 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.mp3'
^C
$ mediainfo РАСПАКОВКА\ _\ UNBOXING\ —\ ОБЗОР\ NINTENDO\ SWITCH-bv3-E92qvxY.webm
General
Complete name : РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm
Format : WebM
Format version : Version 2
File size : 68.3 MiB
Duration : 14 min 22 s
Overall bit rate mode : Variable
Overall bit rate : 664 kb/s
Writing application : Google
Writing library : Google
Video
ID : 1
Format : VP8
Codec ID : V_VP8
Bit rate : 501 kb/s
Width : 640 pixels
Height : 360 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Compression mode : Lossy
Default : Yes
Forced : No
Audio
ID : 2
Format : Vorbis
Format settings, Floor : 1
Codec ID : A_VORBIS
Duration : 14 min 22 s
Bit rate mode : Variable
Bit rate : 128 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Compression mode : Lossy
Stream size : 13.2 MiB (19%)
Writing application : Google
Default : Yes
Forced : No
Но если формат не указан, качается аудио. И очень ме-е-е-дленно:
$ youtube-dl -v -x --audio-format mp3 https://www.youtube.com/watch?v=bv3-E92qvxY&list=PLGJT6m6pbQ2BYzrbAdBsQ1hQ4_JQ9giiB
[1] 1802
$ [debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '-x', '--audio-format', 'mp3', 'https://www.youtube.com/watch?v=bv3-E92qvxY']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.12.23
[debug] Python version 3.5.4 - Linux-4.4.97-std-pae-alt0.M80P.1-i686-with-fedora-ALT-Linux
[debug] exe versions: avconv 11.6-alt1, avprobe 11.6-alt1
[debug] Proxy map: {}
[youtube] bv3-E92qvxY: Downloading webpage
[youtube] bv3-E92qvxY: Downloading video info webpage
[youtube] bv3-E92qvxY: Extracting video information
[debug] Invoking downloader on 'https://r1---sn-cxauxaxjvh-hn9ez.googlevideo.com/videoplayback?ipbits=0&signature=7E94F5DC0FF264E0FF003A5399A03BA82AE3AAA0.DD659D5BFCABD42F32A1553F606E19B05A2482B0&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&initcwndbps=641250&expire=1533760806&lmt=1492862734502537&mime=audio%2Fwebm&key=yt6&itag=251&clen=15368857&gir=yes&keepalive=yes&fvip=1&c=WEB&ei=xQBrW__9M5TKgAea74mwCQ&mt=1533739120&mv=m&ms=au%2Conr&source=youtube&ip=37.212.23.117&mm=31%2C26&pl=19&mn=sn-cxauxaxjvh-hn9ez%2Csn-4g5e6nle&id=o-APa4GgKa5PMZf9_y4Msj3bRuA1raGCeiAgfp_-L0oTiJ&dur=862.941&requiressl=yes&ratebypass=yes'
[download] Destination: РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm
[download] 100% of 14.66MiB in 04:52
[debug] avconv command line: avprobe -show_streams 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm'
[ffmpeg] Destination: РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.mp3
[debug] ffmpeg command line: avconv -y -i 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm' -vn -acodec libmp3lame -q:a 5 'file:РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.mp3'
^C
$ mediainfo РАСПАКОВКА\ _\ UNBOXING\ —\ ОБЗОР\ NINTENDO\ SWITCH-bv3-E92qvxY.webm
General
Complete name : РАСПАКОВКА _ UNBOXING — ОБЗОР NINTENDO SWITCH-bv3-E92qvxY.webm
Format : WebM
Format version : Version 4 / Version 2
File size : 14.7 MiB
Duration : 14 min 22 s
Overall bit rate : 142 kb/s
Writing application : google
Writing library : google
Audio
ID : 1
Format : Opus
Codec ID : A_OPUS
Duration : 14 min 22 s
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 kHz
Compression mode : Lossy
Language : English
Default : Yes
Forced : No