Actually, this should just work as long as you put physics hands in the scene (all the demo scenes use physics hands, so you can use them as a starting point.
Just put a script such as the following on your cube:
using UnityEngine;
using System.Collections;
public class Trigger : MonoBehaviour {
void OnTriggerEnter(Collider other){
Debug.Log("Triggered.");
}
}
And replace the Debug.Log statement with the code that platys your sound. Note that you might get multiple triggers, so you may need some code to prevent the sound from being played more than once at a time.