To mute the Marathon you must first turn it on, then press the mute/flash button on the earpiece. Just a quick press and release. You’ll know the mute is activated when the light on the base next to Mute glows red. To turn off mute, just tap the mute/flash button on the earpiece again.
The Marathon should beep every 15 seconds or so to let you know you’re on mute.
To adjust the listening volume, you would also use the Volume + and - buttons on the earpiece.
To adjust mic volume, you would use the Volume ^ and v buttons on the charging base.
The headset will beep when you raise/lower listening volume, but not when you adjust the mic!