Player Footsteps in stairs Problem

Hi!

I have a problem with my footsteps script when I walk down some stairs ingame. Just watch the video below to see what happens.

Video: https://www.youtube.com/watch?v=fHtyNNV0ELA

Does anyone know why it's like that?

Footstep script:

 public var walkSounds : AudioClip[];
 public var runSounds : AudioClip[];
  
 public var walkAudioSpeed : float = 0.4;
 public var runAudioSpeed : float = 0.2;
  
 private var walkAudioTimer : float = 0.0;
 private var runAudioTimer : float = 0.0;
  
 var isWalking : boolean = false;
 var isRunning : boolean = false;
  
 public var walkSpeed : float = 8;
 public var runSpeed : float = 20;
  
 public var sprintDuration : float = 10.0;
 private var sprintTimer : float = 0;
 private var stamina : float = 0.0;
 var CamAnimationScript : CamAnimation; 
 //public var staminaTexture : Texture;
  
 private var chCtrl: CharacterController;
 private var chMotor: CharacterMotor;
 public var flashlightAnimation : GameObject;
 
  
 function Start()
 {
     chCtrl = GetComponent(CharacterController);
     chMotor = GetComponent(CharacterMotor);
 }
  
 function Update()
 {
     SetSpeed();
  
     if ( chCtrl.isGrounded )
     {
        PlayFootsteps();
     }
     else
     {
        walkAudioTimer = 1000.0;
        runAudioTimer = 1000.0;
     }
 }
  
 function SetSpeed()
 {
     var speed = walkSpeed;
  
     if ( Input.GetAxis( "Horizontal" ) || Input.GetAxis( "Vertical" ))
     {
        if ( Input.GetKey( "left shift" ) || Input.GetKey( "right shift" ) )
        {
 // Running
          isWalking = false;
          isRunning = true;
          CamAnimationScript.running = true;
          CamAnimationScript.walking = false;
        }
        else
        {
 // Walking
          isWalking = true;
          isRunning = false;
          flashlightAnimation.animation.CrossFade ("Flashlight", 0.2);
          CamAnimationScript.running = false;
          CamAnimationScript.walking = true;
        }
     }
     else
     {
 // Stopped
        isWalking = false;
        isRunning = false;
        flashlightAnimation.animation.CrossFade ("Flashlight", 0.2);
     }
  
 // check the sprint timer
     if ( isRunning )
     {
        sprintTimer += Time.deltaTime;
  
        if ( sprintTimer > sprintDuration )
        {
          isRunning = false;
          isWalking = true;
          CamAnimationScript.running = false;
          CamAnimationScript.walking = true;
        }
        else if ( chCtrl.isGrounded )
        {
          speed = runSpeed;
        }
     }
     else
     {
        sprintTimer -= Time.deltaTime;
     }
  
     sprintTimer = Mathf.Clamp( sprintTimer, 0, sprintDuration );
  
     chMotor.movement.maxForwardSpeed = speed;
  
     stamina = 1.0 - ( sprintTimer / sprintDuration );
 }
  
 function PlayFootsteps()
 {
 // Play Audio
     if ( isWalking )
     {
        if ( walkAudioTimer > walkAudioSpeed )
        {
          audio.Stop();
          audio.clip = walkSounds[ Random.Range( 0, walkSounds.Length ) ];
          audio.Play();
          walkAudioTimer = 0.0;
          flashlightAnimation.animation.CrossFade ("Flashlight", 0.2);
        }
     }
     else if ( isRunning )
     {
        if ( runAudioTimer > runAudioSpeed )
        {
          audio.Stop();
          audio.clip = runSounds[ Random.Range( 0, runSounds.Length ) ];
          audio.Play();
          runAudioTimer = 0.0;
          flashlightAnimation.animation.CrossFade ("FlashlightSprint", 0.2);
        }
     }
     else
     {
        audio.Stop();
     }
  
 // increment timers
     walkAudioTimer += Time.deltaTime;
     runAudioTimer += Time.deltaTime;
 }
  
 //function OnGUI()
 //{
 //    if ( staminaTexture ) // check there is a texture so it doesn't error if not
 //    {
 //        GUI.BeginGroup(new Rect(30, Screen.height - 50, 405 * stamina, 25));
 //        GUI.DrawTexture(new Rect(0, 0, 405, 25), staminaTexture);
 //        GUI.depth = 0;
 //        GUI.EndGroup();
 //    }
 //}