Repository: yt-dlp
Issue: Videos downloaded from coub don't loop until the end of the audio track
Labels: site-enhancement

### Checklist

- [X] I'm reporting a broken site
- [X] I've verified that I'm running yt-dlp version **2021.12.01**. ([update instructions](https://github.com/yt-dlp/yt-dlp#update))
- [X] I've checked that all provided URLs are alive and playable in a browser
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/ytdl-org/youtube-dl#video-url-contains-an-ampersand-and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues including closed ones. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

Canada

### Description

Coub is sort of like vine except the audio track can run for longer than the video. The video simply loops for the duration of the audio track. When downloading a coub url, the video only plays for the first time and freezes on the last frame for the duration of the audio track, when it should repeat.

### Verbose log

```shell
[debug] Command-line config: ['-Uv', 'https://coub.com/view/2ynnnr']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, err utf-8, pref UTF-8
[debug] yt-dlp version 2021.12.01 [91f071af6]
[debug] Python version 3.9.6 (CPython 64bit) - Linux-5.14.21-x86_64-with-glibc2.33
[debug] exe versions: ffmpeg 4.4.1 (setts), ffprobe 4.4.1, rtmpdump 2.4
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2021.12.01, Current version: 2021.12.01
yt-dlp is up to date (2021.12.01)
[debug] [Coub] Extracting URL: https://coub.com/view/2ynnnr
[Coub] 2ynnnr: Downloading JSON metadata
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Default format spec: bestvideo*+bestaudio/best
[info] 2ynnnr: Downloading 1 format(s): html5-video-higher+html5-audio-high
[debug] Invoking downloader on "https://coub-anubis-a.akamaized.net/coub_storage/coub/simple/cw_file/4eab6d68b57/02303c8620867fa30760b/muted_mp4_huge_size_1638911654_muted_huge.mp4"
[download] Destination: @candy_balll [2ynnnr].fhtml5-video-higher.mp4
[download] 100% of 2.03MiB in 00:00                                     
[debug] Invoking downloader on "https://coub-anubis-a.akamaized.net/coub_storage/coub/simple/cw_looped_audio_high/ff30f22a4de/1329259b65f42fd114c43/1638911657_mp3-high.mp3"
[download] Destination: @candy_balll [2ynnnr].fhtml5-audio-high.mp3
[download] 100% of 1.07MiB in 00:00                                     
[Merger] Merging formats into "@candy_balll [2ynnnr].mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:@candy_balll [2ynnnr].fhtml5-video-higher.mp4' -i 'file:@candy_balll [2ynnnr].fhtml5-audio-high.mp3' -c copy -map 0:v:0 -map 1:a:0 'file:@candy_balll [2ynnnr].temp.mp4'
Deleting original file @candy_balll [2ynnnr].fhtml5-video-higher.mp4 (pass -k to keep)
Deleting original file @candy_balll [2ynnnr].fhtml5-audio-high.mp3 (pass -k to keep)
```