Chosen Solution

I’ve replaced the front camera flex cable since I torn the previous one, but sound isn’t at the expected level, it’s too low, what could cause that ?

I presume you are referring to the ear speaker. It could be an alignment issue as there is a “nub” on the ear speaker that outputs the sound. It has to sit nicely in the ear speaker mesh so that it channels the sound properly. I would check out this guide to double check you work and insure the ear speaker is properly installed.