iPhone 6s keeps rebooting after screen replacement

One week ago I did a screen replacement on my iPhone 6s. I successfully tested the home button, sensors like the front-facing camera, the speakers and so on.

Two days later the smartphone didn't boot any more. When connecting it to the charger the Apple logo appeared on the screen but after a few seconds it turned off again. I started to remove flex cables to find the origin of the problem. After removing the first cable (the cable of camera, speaker, sensors) it booted again. I reattached all the connectors I had unplugged before and put the screen onto the body. It worked.

I thought the issue was generated by a loose flex cable or connector. So I tested if there were any loose connectors by lightly hitting it once with my forehand. It turned off again.

In my opinion the top speaker came loose because it is connected with just four contacts which are loosely held in position by two small pins.

I had to cut this piece of tape because it didn't fit. This could also be the reason for this issue. Shown in this image:

https://imgur.com/a/68EXslK