Footsteps script (counter version). Attach this to the first person controller and add an AudioSource component.

Footsteps.cs

using UnityEngine;
using System.Collections;

public class Footsteps : MonoBehaviour {
	public AudioClip[] footsteps;
	public CharacterController controller;
	
	float stepTimer = 0;
	int stepCounter = 0;
	
	void Update() {
		if(controller.velocity.sqrMagnitude > 10 && controller.isGrounded)
		{
			stepTimer = stepTimer + Time.deltaTime;
		}
		else
		{
			stepTimer = 0;
		}
		
		if(stepTimer > 0.25f)
		{
			audio.PlayOneShot(footsteps[stepCounter]);
			stepTimer = 0;
			stepCounter++;
			
			if(stepCounter > footsteps.Length-1)
			{
				stepCounter = 0;
			}
		}
	}
}