In practice this affects only the GTK (and WPE I suppose) ports because those are the only ones calling muteChanged() but the change is for the cross-platform MediaPlayer. This bug is related to bug 174241 which has some more explanations.
Created attachment 331331 [details] Patch
Committed r226948: <https://trac.webkit.org/changeset/226948>
Thanks. I'll notify all those gardening bugs...
<rdar://problem/36521946>