It might be worthwhile to note that even with the startup muted (or external speakers plugged in) if your newer (OSX compatible) Mac has a hardware problem, the internal speaker will still send the series of beeps that (kind of) help you diagnose the problem.
That's why disabling the internal speaker is not a good solution.
Most users have never heard them, and thats A Good Thing.
The startup chime was originally a hardware diagnostic tool; with older hardware instead of the dimminished 7th chord which represents a "pass" you would hear each note of the chord played in series, representing a "fail". (Bad memory is the most common cause). Newer Macs use a different system, so the chime is redundant now.