Hi
Either slide the button (Track Volume) at the left hand end of the Audio Track to the extreme left to show 0% or click the blue loudspeaker icon above this (pointed to by Borate) . The track becomes chequered when muted.
As an alternative you can right click the audio track and select Unlink from Video then right click it again and select Delete
This will remove the audio track leaving a blank track for you to add your own.
Nat