How to use audiosource in c# in Unity 4? Did it change?

In a Unity 3 test game I did the following to play a sound....

 AudioSource gmMus;
 
 public AudioClip music;
 
 void Start()
 
 {
 
  gmMus = (AudioSource) GetComponent(typeof(AudioSource));
 
 }
 
 void Update()
 
 {
 
   gmMus.PlayOneShot (music);
 
 }
 
 The above worked fine in Unity 3.
 
 In Unity 4 the same code does not work.
 The ' gmMus = (AudioSource) GetComponent(typeof(AudioSource));' fails and NULL is assigned to gmMus.
 
 Why this change?
 
 In the Unity 4 documentation I now see the following:
 
 using UnityEngine;
 
 using System.Collections;
 
 [RequireComponent(typeof(AudioSource))]
 
 public class example : MonoBehaviour {
 
     public AudioClip impact;
 
     void OnCollisionEnter()
     {
         audio.PlayOneShot(impact, 0.7F);
     }
 }

This new sample code confuses me. How did the var 'audio' get assigned for 'audio.PlayOneShot(impact, 0.7F);' to work?