Basically: Add trigger > When the > Display > Screen Lock > is > on > then > Run the Shortcut > Mute Computer
In the add trigger sheet, click Display, then Screen lock.
Click On and then create new shortcut
Contents of shortcut is below, you drag the Actions in from the right.If you name it and close it, then switch back to Trypa, it will Automatically be selected. Then press Save in Trypa and close the preferences panel.