I looked at the video and was not able to determine exactly which sound effect was the hit sound effect. It could be that the one you chose is getting drowned out by others in the scene at the time, but to me listening on earbuds I could not pick it out..
As for the health bar, my immediate reaction would be to add some sort of flash/pulse to the health bar when the player is low on health (refer to Halo’s shield bar when shields are low/broken). Alternatively you could do a pulse effect on the screen itself (I think Skyrim does this).
However, going back to the previous comments, anything you do to the health bar might not matter if there is too much happening on screen. For example, at around the 2:30 mark in the video you provided I noticed that there were a lot of visual effects going on at one time. Someone who is easily distracted might get caught up in the action on screen and not notice the health bar.
Side note: I don’t remember seeing any gameplay where the player dies or has extremely low health. It is possible you have accounted for the things I have mentioned but it just didn’t happen to show in the gameplay.
To summarize:
- see if you can mix the audio to allow the hit effect to be more prominent
- Investigate adding some visual cue to the health bar/screen
- Investigate ways to reduce visual effects on screen where unnecessary
I hope this helps.