As pointed out by slomo :) MediaPlayerPrivateGStreamer.cpp in setMuted() it should really use the "mute" property of playbin2. That keeps the current volume and just mutes the audio ;)
Created attachment 40438 [details] proposed patch
Comment on attachment 40438 [details] proposed patch Very good.
Comment on attachment 40438 [details] proposed patch Clearing flags on attachment: 40438 Committed r48980: <http://trac.webkit.org/changeset/48980>
All reviewed patches have been landed. Closing bug.