Only able to play a single sound through AudioSource.

I am trying to set up my script so that it will choose randomly from three sounds when I fire the weapon. It works if I set it up with one sound but fails to play anything when I put in my if statement to attempt to randomise the sound. I've been poring over the manual entries on AudioSource and AudioClip all morning and have not been able to figure this out.

 public AudioClip launchMissile1;//audio for firing missile
 public AudioClip launchMissile2;//audio for firing missile
 public AudioClip launchMissile3;//audio for firing missile
 
         //capture keyboard input to launch a starbase missile
         if(Input.GetKeyDown(keyFire))
         {
             if(weaponLoaded)
             {
                 FireWeapon();
                 float randSound = Random.value;
                 if(randSound < 0.33)
                 {
                     print ("sound1");
                     audio.clip = launchMissile1;
                     audio.Play();
                 }
                 if(randSound >= 0.33 && randSound <= 0.66)
                 {
                     print ("sound2");
                     audio.clip = launchMissile2;
                     audio.Play();
                 }
                 if(randSound > 0.66)
                 {
                     print ("sound3");
                     audio.clip = launchMissile3;
                     audio.Play();
                 }
                 rigidbody.AddRelativeForce(Vector3.up * recoil);
             }
             else
                 print ("We're still loading the launcher!"); //debugging info            
         }

So the question at it's most basic is "How do I change the clip that my audiosource is playing?".

Thanks in advance for any assistance.