The noise is only there when VoiceOver is turned on, I could not get the power-off sound to play with VoiceOver off. Furthermore, the device has to not be on mute.
But it seems iOS 16.1 has a feature for the blind, so we know when our phones are turning off. There's no power-on sound effect however, either with or without VO--at least not on my iPhone 13.
The only devices I know of to have both sounds are the iPhone 14 series. If your older device doesn't have the sound for power-off when using iOS 16.1 with the device unmuted, it would be interesting to know which.
Fluke? Or am I right in thinking this was left there for VO users on purpose?
It's interesting that there's no power-on sound, but us blind people will know when our phones are turned on anyway by VoiceOver speaking. So I think this is just more a useful feature than a flashy sound.