Update the video codecs.
Deb-package
Install the latest version of chromium-codecs-ffmpeg-extra by running sudo apt-get update && sudo apt-get install chromium-codecs-ffmpeg-extra.

Rpm-package
Download the latest version of chromium-ffmpegsumo, clicking the package name in the Download column.

Install the package by running:

sudo rpm -i <path-to-downloaded-file>/chromium-ffmpegsumo-<version-number>.rpm
Other distributives
Download the latest available deb-package.
Extract the libffmpeg.so file from the archive:

!/bin/bash

ar -x chromium-codecs-ffmpeg-extra_56.0.2924.76-0ubuntu0.16.04.1268_amd64.deb
rm chromium-codecs-ffmpeg-extra_56.0.2924.76-0ubuntu0.16.04.1268_amd64.deb control.tar.gz debian-binary
tar xf data.tar.xz
mv usr/lib/chromium-browser/libffmpeg.so
rm -rf usr data.tar.xz
Put it in the library /opt/yandex/browser-beta/lib/libffmpeg.so.