I don't observe any specific pattern of this behaviour. Strangely, sometimes when my macbook reboots/boots, it plays a chime sound.
And often in other cases, no chime sound is played.
So what determines whether to play a chime sound?
It depends on the volume rate before you Restart or Shut Down your mac
so if your volume level is 0 (Mute), you will hear no chime sound on start-up, just try it and make sure
I have Muted my mac speakers because i always use headset (as you know you can set headset volume separately when you connect it) so i never hear a chime sound,
i searched a lot and found this (muting) as a solution because i hated that voice (imagine restarting in the middle of the night!)
hope this helps
According to wikipedia.org,
So your Mac might have some issues, which can be resolved by resetting the PRAM. You can perform this by holding CMD-OPTION-P-R while your Mac is booting. Hold the key down until you hear the chime two times. Then release the keys.