When a creature moves while wearing metal armor, it will produce clear armor clinking sounds. Heavy steps, metallic movement, and constant rattling make armored mobs feel more physical and alive.
And if that creature is a player, the noise is not just cosmetic. Nearby hostile mobs can notice it.
Wearing metal armor now has a real gameplay cost: better protection, but less stealth. Moving around in full gear may draw unwanted attention, especially when enemies are close enough to hear you.
Customization can be made with datapacks and resourcepacks. Check wiki for more information.
Credit: the sound source of metal armor is made by pilotlala, shared on freesound.org; the sound source of crystal armor is made by basharov, shared on freesound.org.