I've got Norton Antivirus running on my PC and it has been dutifully disabling some files that it detects in the Mist download when I unzip it. In re-enabling these files, I noticed that ffmpeg.dll got detected and disabled. That's an MPEG library, just curious why it was included in Mist. Audio?
ffmpeg lib is not due to Mist but to chromium that holds the Mist Dapp.
Mist is a web application (html/JS/CSS) packaged within a Chromium browser as a single executable for more convenience. But in a near future, you will probably be able to use Mist to run any Dapp.
Have a look at LICENSES.chromium.html at the root of Ethereum Wallet archive and you'll notice that ffmpeg licence is listed here.