I would suggest a state machine, but that's a bit more than what's probably needed for this problem. Maybe you could have a variable which is assigned when the player lands on the floor. After that, check for whether the sound was played. It could look something like this:
func _physics_process(delta):
if Velocity.length() == 0 and is_on_floor() and not is_on_wall() and not sound_has_played:
sound_has_played = true
$Landing.play()
Once the player jumps, assign false
to sound_has_played
.