Loud speaker doesn't work

Hello, the loud speaker of my iPhone 4 doesn't work. The iPhone dropped months ago, so I bought a new one because I needed it. Few days ago I bought a new screen/digitizer for the broken one and replaced it. Everything is working fine except the loud speaker.

Things I tried:

- Replaced Speaker Assembly

- Replaced Dock Flex

It did not work. If I try to Volume up/down my iPhone shows the normal internal Volume Bar, so it's not the Headphone Jack "Bug".

Next one I did was taking apart the Phone and testing the Speaker itself. I connected the Speaker to some MP3 Player Output and it works. Next thing was to test the connector on the dock flex cable. I used another speaker and connected the pins directly to the iPhone pins (located at the flex) and there was an output.

So, actually I don't know what's wrong. According to some wiring scheme from the internet, the iPhone has a preamp. For me it looks like the preamp is located at the flex cable, so it's already replaced with a new one.

After this I was thinking, ok, it's an connection problem between speaker and flex. But it's not. I removed the dock flex and attached it temporarily without installing it. Also attached the speaker to the flex temporarily and it doesn't work.

What can cause this problem? Any ideas?

Thanks and regards