Bulwark has 2 sound effects and 2 particle effects which are used based on what the attacking unit is.

If it is a hero, ward, fountain, or a spirit bear, it uses the louder sound and flashier particles.

If it is any other unit (creep, summons, roshan, neutrals, towers), it uses the quiet sound and smaller particle effects.

So what we have is, it uses the heavy sound vs Plague Wards (which deal 13 damage on level 1), but uses the quiet sound vs Roshan (75 dmg), Warlock's Golems (100-200 dmg), Necronomicon summons (120/125 dmg), or any creep with a heavy damage boost buff (e.g. Alacrity or Divine Favor).

A weak support hero who deals 20 damage after armor reduction should not trigger the same loud block sound as a 1000 damage crit from a farmed PA.

A Plague Ward dealing 13 damage should not trigger the heavy sound, while Roshan dealing 100 dmg per attack triggers the weak sound.