StopSound not working on certain .SNT

[Damascus]

I'm having lots of trouble getting a sound to stop in one of my maps. When the player enters a certain area, I want a prisoner chained to the wall to start muttering. However, since the muttering has such a wide range I want it to stop when the player leaves the area, but the sound won't stop.

"Prisoner" is the offending sound:

PHP Code: (Select All)

void OnStart{
    AddEntityCollideCallback("Player", "TerrorArea", "CollideTerrorArea", false, 0);
}

void CollideTerrorArea(string &in asParent, string &in asChild, int alState)
{
    if(alState == 1)
    {
        FadeSepiaColorTo(0.75f, 0.7f);
        FadeRadialBlurTo(0.025f, 0.02f);
        AddTimer("terror", 0, "TimerTerrorDrain");
        SetLocalVarInt("InTerror", 1);
        PlaySoundAtEntity("Prisoner", "24_mb_02.snt", "chained_prisoner_3", 1.0f, true);
    }
    else if(alState == -1)
    {
        FadeSepiaColorTo(0.0f, 1.0f);
        FadeRadialBlurTo(0.0f, 0.02f);
        RemoveTimer("terror");
        SetLocalVarInt("InTerror", 0);
        StopSound("Prisoner", 1.0f);
    }
} 


Every other script works when the player leaves; Sepia and Radial Blur return to normal, but the muttering continues. When the player then re-enters the area, a second muttering sound will start and overlap the original.