My Brothers iMac ignores that it was muted before shut down and makes the start up sound when it will be started again. This happened all of the sudden – normally you can avoid the start up sound by muting the Mac just before the shut down.
On my MacBook Pro everything works fine and I can start noiseless. I have macOS Sierra (10.12.6) and he has macOS Sierra (10.12.4). I can't imagine that the reason is the older OS. If the OS would be the reason it would make more sense if it was the other way round because Apple already killed some options to mute the start up sound before.
Does somebody has an explanation for this?