You could try using bool variables to act as switches - so if W is pressed, set a bool ‘MyWBool’ to true. And when it’s not pressed set it to false. Then you could do something similar with the trigger box - if the actor has entered the trigger box set ‘MyTriggerBool’ to true, and when you leave the trigger set it to false.
Then all you have to do is check every frame(tick) if the W key is pressed and if the player is in the trigger box (branch node) and if both are true then you can play your footsteps sound. If it’s not true then don’t play/stop playing the sound.
If you hit the F key and go into fly mode do you start hovering above the ground? If so then the above applies as you could have a trigger box just covering the floor so when your feet leave the ground it will set ‘MyTriggerBool’ to false and so pressing the W key won’t play the footstep sound.