For a while now Flash player for Firefox is upmixing stereo sound to all device channels. So if you happen to have a multichannel sound device (let’s say 5.1 HDMI audio connection to A/V receiver), you are in for a surprise remix of your favorite videos.
I’ve been quite annoyed by this “feature” of our beloved Adobe for a while now.There is no way to alter this behavior, and the upmixing itself is done badly. Today I found a solution posted on Superuser StackExchange by user e-t172. And it works!