Hello,
The problem you are describing is likely caused by liquid damage on your dock (the part that handles the speakers, microphone, antenna and charging port).
You can check if this port was in contact with water by tearing down your phone using this guide, and spoting any water signs : [guide|50399]
If you’re not sure about that, you can always bring it to a repair store where they will test and make sure the problem is coming from here.
Cheers !