Problems when changing the Earpiece speaker

Hey everyone,

My girlfriend's iPhone 6 had a poor earpiece speaker volume, so I decided to buy a new earpiece speaker and change for her. I have experience fixing iPhones, so I got to the broken part with no trouble. After changing it and reassembling the problem is not completely gone. I also noticed that when the phone is open and without the metal plate that holds the flat cables, the sound that comes from the earpiece speaker is very loud, perfect, and when I put the metal plate in place and close the phone the sound is very low again.

I tried to be as clear as possible.

What can I do?