Mel,
Did you consider using one of Instant Messengers for that?

>Does anyone know a way to cause a "beep" or similar alert to sound, or somehow a message to appear on another persons computer who is connected to the internet? In other words, one person needs to immediately notify about 60 other people who are connected to the internet to look at their monitor. Time is important, within like 1 or 2 seconds.
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>This would only be done on a persons computer that expects and needs to hear this notification, and the ip address of each machine is known.