How would i add different surfaces? FOOTSTEPS

My footsteps now only plays a walk sound and run sound. But how do i make a walk sound for Metal, run sound for metal and walk sound for concrete, run sound for concrete?

 #pragma strict
 
 var walk : AudioClip[]; // fill this array with the sounds at the Inspector
 var run : AudioClip[]; // fill this array with the sounds at the Inspector
 
 var walkAudioSpeed : float = 0.4;
 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;
 
 var chCtrlReturnTrue : boolean = true;
 
 var clipType : String = "";
 
 function Update()
 {
     if ( chCtrlReturnTrue )
     {
         PlayFootsteps();
     }
     else
     {
         walkAudioTimer = 0.0;
         runAudioTimer = 0.0;
         audio.pitch = 0.9 + 0.7*Random.value; // randomize pitch in the range 1 +/- 0.1 
     }
 }
 
 function PlayFootsteps() 
 {
     if ( Input.GetAxis( "Horizontal" ) || Input.GetAxis( "Vertical" ) )
     {
        if ( Input.GetKey( "left shift" ) || Input.GetKey( "right shift" ) )
        {
          // Running
          isWalking = false;
          isRunning = true;
          
        }
        else
        {
          // Walking
          isWalking = true;
          isRunning = false;
          
        }
     }
     else
     {
        // Stopped
        isWalking = false;
        isRunning = false;
        clipType = "";
        
     }
 
     // Play Audio
     if ( isWalking )
     {
        if ( clipType != "walk" )
        {
          audio.Stop();
          audio.clip = walk[ Random.Range(0, walk.Length) ];
          clipType = "walk";
        }
 
        //if ( !audio.isPlaying )
        if ( walkAudioTimer > walkAudioSpeed )
        {
          Debug.Log("Playing");
          audio.Stop();
          audio.Play();
          walkAudioTimer = 0.0;
        }
     }
     else if ( isRunning )
     {
        if ( clipType != "run" )
        {
          audio.Stop();
          audio.clip = run[ Random.Range(0, run.Length) ];
          clipType = "run";
        }
 
        if ( runAudioTimer > runAudioSpeed )
        {
          audio.Stop();
          audio.Play();
          runAudioTimer = 0.0;
          //You can randomize the audio clip again here, if you need to 
        }
     }
     else
     {
        audio.Stop();
     }
 
     // increment timers
     walkAudioTimer += Time.deltaTime;
     runAudioTimer += Time.deltaTime;    
 }