You don't have to follow standard ways of doing things (although it's almost always best when there is one) - sometimes a standard simply don't exists - as in this case.
What you need is a place where people can mute (button, menu or whatever) and a text that explains what the shortcut is. This can be either a adjacent text label, mouse over popup, or a place where all the shortcuts are gathered (like Photoshop does -
(source: photoshopessentials.com) ).
Shortcuts should be used as the name describes it - a shortcut for a function that the user can use without having to know about the shortcut - hence the button or similar that mutes the sound...
If the shortcuts are used or not used that much is another question, but the ones who uses them appreciates them - and I am one of them :)
downare good shortcuts for adjusting volume in a media player (see youtube), you could extend this to include shift+up and shift+down for full volume and mute.