In version 1.7.2 zombies cannot break wooden doors on any difficulty, doors have no visible damage and do not produce the sound of breaking. All happens like it's EASY difficulty.

How to reproduce:
1) Set difficulty to HARD
2) Build a house with a wooden door
3) Spawn a villager inside
4) Set night time (or build an overhang)
5) Spawn a zombie outside house and wait.