Program gets stuck on ffmpeg conversion

[NightMachinary]

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I first noticed this problem when using instantmusic: yask123/Instant-Music-Downloader#126
But the problem is with yt-dl:

youtube-dl --audio-format mp3 --prefer-avconv -x https://www.youtube.com/watch?v=3oJxBhD94_s
[youtube] 3oJxBhD94_s: Downloading webpage
[youtube] 3oJxBhD94_s: Downloading video info webpage
[download] Destination: Melanie Martinez - milk and cookies (ONE HOUR LOOP)-3oJxBhD94_s.webm
[download] 100% of 67.26MiB in 00:02
[ffmpeg] Destination: Melanie Martinez - milk and cookies (ONE HOUR LOOP)-3oJxBhD94_s.mp3

It just hangs on this. I tried forcing it to use avconv, but with no success. I tried using m4a, but then I get:

[ffmpeg] Destination: Melanie Martinez - milk and cookies (ONE HOUR LOOP)-3oJxBhD94_s.m4a
ERROR: audio conversion failed: [aac @ 0x682100] The encoder 'aac' is experimental but experimental codecs are not enabled, add '-strict -2' if you want to use it.

[NightMachinary]

It eventually unstuck. After a long 10 minutes.

[Hrxn]

Outdated ffmpeg?