It is in fact very simple: All you need to do is attach an AudioSource to the same game object (or any other game object), provide a "slot" for that audio source, e.g.

var audio : AudioSource;

And then call

audio.PlayOneShot(audio.clip);

in your if-blocks. If you want multiple sounds, simply create one game object with relevant audiosources attached for each sound you want to play (and create var audio1 : AudioSource; and so on, or use names that actually will tell you what the sound means).

uhmm got a little problem where to place the audio.PlayOneShot(audio.clip); ??? please help this sad guest

var model : GameObject;
var track : AudioSource;

function OnGUI () { 

  //background box 
  GUI.Box (Rect (10,130,100,90), "Sounds");

  //Make first button
  if (GUI.Button (Rect(20,160,80,20),  "Go")){
   audio.clip = track;
        audio.loop = false;

      audio.Play();

  } 

  //Make second button
  if (GUI.Button (Rect(20,190,80,20),  "Stop")){ 

      audio.Stop();

  } 
}

uhm really thanks but... i got a error and i dont know what to do can you help me ?

Assets/side-scroller/Scripts/GUI Options Button.js(40,25): BCE0022: Cannot convert 'UnityEngine.AudioSource' to 'UnityEngine.AudioClip'

what to do ?

var track : AudioClip;