Damaging yourself whilst wearing Thorns enchanted armour will not produce the Thorns sound when you take damage.

Steps to Reproduce:
1) Wear thorns armour and shoot yourself
2) Look at subtitles as you take damage
3)When you get the death message "<player> was killed trying to hurt <player>", confirming that you took damage from thorns, you will notice that there was no subtitle or sound mentioning the "Thorns prick" damage