I think earlier versions (around System 3) even supported a custom "StartupSound" file that would play while the machine started up. But you cannot replace the boot chime, its burnt into ROM.

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Never heard of that. Systems before System 7 could not play sounds natively (outside of the system beep, of course, which was located in the System file). With System 7, you could double-click a snd file and hear it play. When QuickTime came around in 1991, placing a StartupMovie in the System Folder would cause it to play over the Welcome to Macintosh dialog.