You can set your Power Mac to mute, but I wouldn't want to turn it off, it's how you know that the initial POST has been successful; if problems begin to start and you switch off the chime, it might leave you wondering how far it's getting during boot.
Like lil stated the start up chime, will tell what is wrong with your system if there is one. There are different sounds for each problem. Like if there is no memory or bad memory, you get a different cord.
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