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 Example() {
        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 Example() as IEnumerator:
        audio.Play()
        yield WaitForSeconds(audio.clip.length)
        audio.clip = otherClip
        audio.Play()