AudioSource.clip  

var otherClip: AudioClip;

// Play default sound
audio.Play();

// Wait for the audio to have finished
yield WaitForSeconds (audio.clip.length);

// Assign the other clip and play it
audio.clip = otherClip;
audio.Play();

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public AudioClip otherClip;
IEnumerator Awake() {
audio.Play();
yield return new WaitForSeconds(audio.clip.length);
audio.clip = otherClip;
audio.Play();
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public otherClip as AudioClip

def Awake() as IEnumerator:
audio.Play()
yield WaitForSeconds(audio.clip.length)
audio.clip = otherClip
audio.Play()