Why my macbook sometimes plays a chime when booting/rebooting, sometimes it does not?
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