There is no keyboard shortcut assigned to the Mute process by default. However, one can be created under "Options> Customize Keyboard". In the 'Show commands containing' box, type 'Mute'. Click the 'Process> Mute' result and type in your desired keyboard command and click the 'Add' button, then 'OK'.
If your desired command is already assigned to another process, it will state so in the bottom box: 'Shortcut currently assigned to'. You can enter a subsequent choice or.. the assigned command you desire can be removed or changed from the default, freeing it up for re-assignment.
Addendum; You can also place a Mute icon in toolbar as well. Same for most of the other commands. "View> Toolbars> Customize".