In the previous android version (Lollipop, Marshmallow, Nougat and may be Oreo) there was a metronome sound when the phone was waiting for disc encryption password: "To start Android, enter your password".
It was very useful, as it indicated that user action is required.
But now, Pie (and may be Oreo, I don't remember) it's gone. I don't know the behavior of android 10.
Is it possible to bring the metronome sound back?
The older Android versions were using Full Disk Encryption (FDE) mode, therefore the password enter dialog was a small mini-OS booted before the actual Android-OS. This mini-OS was playing what you are a calling the "metronome sound" when it was waiting for the user password.
I assume this sound was introduced to ensure the user recognizes that the boot process requires user interaction. Because while the mini-OS is waiting for input the whole system is offline and no app is running - including no mobile voice/data or Wifi.
Nowadays Android use File Based Encryption (FBE) which means that every app can specify if a data is encrypted using the system key or the key that requires the user to unlock the device first. Therefore in theory if all apps are correctly implemented a large part of the system should already work before you enter your user password the first time after a reboot.
May be you are able to simulate the old behavior by installing an app that runs only after the Android device has booted up (BOOT_COMPLETED) and then plays a sound similar to your old system. Unfortunately I don't know a suitable app, but I assume programmable scripting apps like "Tasker" may allow you to create a script that plays a sound after the device has booted up.
