option #1
Go to the town of Agia Marina in Stratis and find the drainage ditch that splits the town in half. At this drainage ditch, if you run along the concrete walls that make the ditch, there is no footstep sounds played.
Option #2
Find a decent sized boulder and walk on top of it and no footstep sounds can be heard.
Option #3
Destroy a building or structure down to rubble and walk on top of the actual rubble object that takes it place.