The seals between the door speaker and where it’s attached to the door have been known to fail on the mk7/ mk7.5 Golf.
If / when this happens, when it rains or the car’s washed, water entering the door cavity between the window and window rubbers can run into the car around the speaker seal, rather than running out of the drain holes at the bottom of the door and outside the car onto the ground as it should do.
You could test to see if this is the cause - with the door open, get someone to pour water over the outside of the window and let it run into the door cavity. Check to see if all the water is exiting the door through the drain holes at the bottom of the door, or if some is dripping out between the door card and the door or around the speaker grille.