Here's kind of a long roundabout way to get there, it'll work, but it's not free.
Get any of the 3 "Bars" and create/assign an Icon to Mute.
Next, get
VITO ButtonMapper. With VBM you can create a Macro (of sorts) that "Records" a single, or a series of Screen Taps, which you can then assign to a Hard Button either as a Normal Button Press or as a Press & Hold......on any of your Hard Buttons.
For instance, I assigned MagicSS Screen Shot app to my Record Button as a Press & Hold. I have to conciously
want to take a screen shot. I don't have to worry about a whole lot of .bmp files stuck in my Windows Directory from accidental Button Presses
and I don't have to wade through any menus to take a screen shot.........it's right there on the side of my Axim, ready & waiting.
Like I said, it's a roundabout way to get there, but it'll do what you want.