Footsteps with randomness. 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 lastIndex = 0;
	
	void Update()
	{
		if(controller.velocity.sqrMagnitude > 10 && controller.isGrounded)
		{
			stepTimer = stepTimer + Time.deltaTime;
		}
		else
		{
			stepTimer = 0;
		}
		
		if(stepTimer > 0.35f)
		{
			int index = Random.Range(0, footsteps.Length-1);
			
			while(index == lastIndex)
			{
				index = Random.Range(0, footsteps.Length-1);
			}
			
			audio.PlayOneShot(footsteps[index]);
			stepTimer = 0;
			
			lastIndex = index;
		}
	}
}