@script RequireComponent(AudioSource)
public var otherClip: AudioClip;

// Play default sound
function Start()
{
    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;

[RequireComponent(typeof(AudioSource))]
public class example : MonoBehaviour {
    public AudioClip otherClip;
    IEnumerator Start() {
        audio.Play();
        yield return new WaitForSeconds(audio.clip.length);
        audio.clip = otherClip;
        audio.Play();
    }
}

import UnityEngine
import System.Collections

[RequireComponent(AudioSource)]
class example(MonoBehaviour):

    public otherClip as AudioClip

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