When talking on the phone, a weird sound (like a robot dying) comes through the earpiece.

Apple's mobile devices will play an alert sound through the earpiece when your battery reaches 20% and then 10%, but only when the display is off. iOS or iPadOS will play the alert sound because when talking on the phone with the screen off, you are unable to see the on-screen alert notification.

No solution necessary. The phone or tablet is performing as expected.