In order to hear the start up chime, the volume has to be changed at least once before it is shut down and it cannot be muted. The next time you start up, you should hear the sound.
reset your PRAM by holding "command-option-P-R" on startup and keep holding until it boots again. this will set your chime to full volume. then turn your volume down and restart again. it should go back to an acceptable level.