problem about ALERT or PLAYSOUND

 
Hi,
I wrote an indicator. Which includes alert:
......
Alert(Symbol(),"--- BREAKS");
if(allow_sound) PlaySound("alert.wav")
......


Every time when price triggers, there would pop up the alert window and play a sound .
But if a popup window and sound accurs and I don't look at it, (e.g. when I'm not home), next time when price triggers, there will probably be no sound !

Why does that happen?

Reason: