You might want to use PlayClipAtPoint for this. It's great for one-off sound effects which can occur at an arbitrary position in your scene.

How you do this might depend on how you're detonating the barrel. If you have some function on your barrel which handles the explosion, you might do:

var explosionClip : AudioClip;

function Explode() {
   AudioSource.PlayClipAtPoint(explosionClip , transform.position);

   // (then whatever code generates the explosion)
   Destroy(gameObject);
}

You'd then need to make sure you drag in a reference to the audio clip, into the "explosionClip" variable in your barrel object (or prefab).