Is the audio stream set to loop?

Probably you are executing this code into a loop (_physics_process or _process) and move_and_collide is returning the colision information when it's hits the platform, so $AudioStreamPlayer.play() is always be called. To solve it you can declare a gloal variable thats saves the current touched plataform then check if the current platform is diferent from the saved, so the play function will be called once per platform. Something like this:

var current_platform = null # global variable
[...]
func _physics_process(delta):
     [...]
     var colision = move_and_collide(direction.normalized()) # get collision data
     if colision: # check if is colliding
          if current_platform != colision.collider: # check if the colliding object is different from last one
               current_platform = colision.collider # save current platform
              $AudioStreamPlayer.play() # play the audio

Will be good to set current_platform as null when an action like jump be done for the audio be played when platform be touched again.
There are several other approaches to solve this problem, this is one of them.