Only way to mute is to turn the volume down all the way. Though practically you only have to go down to 3 or so before you really can't hear the radio at speed.
You could also press the 'Mode' button and switch to some input without sound (isn't there a 'weather' selection?). Then switch back to the desired source to unmute.
If you truly want to find a way to mute things, you'll have to do some wiring. Either wire a cutoff switch into the speakers somehow or use the GPS input. It overrides the radio (i.e. for GPS nav voice instructions). You could find some way to pipe in 'silent' audio when you press a button, I suppose. These two ideas are pure wild-ass suggestions without much forethought.
2012 Victory Cross Country - Imperial Blue
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2003 Honda VTX 1800C - Illusion Blue