The bug
When an armadillo rolls up, no sound plays despite there being a sound in the game for it (mob.armadillo.roll).
Steps to reproduce
Create a world with the 'Armadillo and Wolf Armor' experimental toggle turned on
Run '/summon armadillo'
Sprint near the armadillo so that it rolls into a ball
Listen to the armadillo and note there is no sound
You may hear a different armadillo sound when it rolls up (namely the landing sound) - note that this is a different sound.
There is a sound in the game for an armadillo rolling up which can be accessed using this command:
/playsound mob.armadillo.roll
Can confirm.