// when any AudioSource goes trough this transform, it will set it as 'mono'
// and will restore the value to 3D effect after exiting
// Make sure the audio source has a collider.     
function OnTriggerEnter(other : Collider) {
     if(other.audio)
        other.audio.spread = 0;
}

function OnTriggerExit(other : Collider) {
     if(other.audio)
        other.audio.spread = 360;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnTriggerEnter(Collider other) {
        if (other.audio)
            other.audio.spread = 0;
        
    }
    void OnTriggerExit(Collider other) {
        if (other.audio)
            other.audio.spread = 360;
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnTriggerEnter(other as Collider):
        if other.audio:
            other.audio.spread = 0

    def OnTriggerExit(other as Collider):
        if other.audio:
            other.audio.spread = 360