It will almost certainly be the speakers and as noted above some of the door card clips can leak a little as well so I would have plenty of spares.
I've done this repair on two mk7's now and I took a more 'ghetto' approach - I didn't remove the speakers and just used a low modulus silicone around the perimiter of each speaker. 'Normal' bathroom grade silicon didn't work very well but the low modulus stuff did. Its not pretty but has worked fine for at least a couple of years.
This was the stuff I used
https://www.halfords.com/motoring/paints-and-body-repair/adhesives/granville-black-silicone-sealant-40g-173867.html
This is pure genius, thanks so much for sharing this. Mine is still fine but I'm sure the day will come and I don't fancy drilling out the rivets.
