Chromium is web browser released by Google Inc, it is the base of Chrome browser (close source).
Chrome come with ffmpeg which can decode mp3 and h.264 movies which is commonly used in HTML 5 video sites like Youtube.
But due to license issue, even the ffmpeg source is came with Chromium, it is disabled in build time.
In case you don't care about the license and you won't distribute your build, you can enable it for video support in HTML5
Before running gclient sync:
export GYP_DEFINES="ffmpeg_branding=Chrome werror="
After running gclient sync:
edit src/net/base/mime_util.cc and comment out those CHROME related defines.
Now you can go to http://www.youtube.com/html5/ to enable the HTML5 version of Youtube, in my computer, the HTML5 version run even smoother than flash!