My thoughts @TonyVit1 are that a start up sound were really on phone's supplied directly by a Network i.e branded. Is this the case with you?
Maybe the Network has asked Samsung to remove that if so.
There are no start up sounds on a Samsung phone supplied directly by Samsung.
Try removing your sim card and re insert to see if that kock starts it. If not then it may have been removed.
I assume all your other system sounds are working as they should ?
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.