Thanks for the feedback !
To be honest, it was 3am (2 hours before deadline) when all systems were completed and all that was left to do (except export) was balancing. That was our worst mistake as trying to tweak something like 30 variables (spawn rate, spawn progression, max hp, hp per hit, coal usage, pressure inc rate, ...) takes a long time and is very difficult when you're half dead inside. XD
It seems the hit sound slipped away. Totally forgot about it.