Is the audio clip looping?

If you disable looping on a playing AudioSource the sound will stop after the end of the current loop.

	@script RequireComponent(AudioSource)
	// Stop looping the sound
	function Start()
	{
		audio.loop = false;
	}

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
public class Example : MonoBehaviour {
    void Start() {
        audio.loop = false;
    }
}

import UnityEngine
import System.Collections

[RequireComponent(typeof(AudioSource))]
public class Example(MonoBehaviour):

	def Start() as void:
		audio.loop = false