It seems obvious to me, but...

If you have a piece of aluminum that has no hole in it, it is waterproof due to its intrinsic fluid permeability. In fact, aluminum foil only 12um thick has a water vapour transmission rate of only 0.2 g/m2/day.

It basically doesn't matter what you do to this piece of aluminum. Unless you smash it with a hammer, it is going to stay waterproof.

If you add a 3.5mm hole to the aluminum, it becomes perfectly permeable. In order to achieve a waterproof state, that hole needs to be filled in. Unfortunately, we can't just fill it in with aluminum because we need a conductive metal to be accessible through the hole. This implies that there will have to be at least two materials involved.

Theoretically, since the conductive material will also be metal, and therefore have a very low WVTR of its own, we could make the jack receiver the exact same size as the hole and thereby fill the hole and retain our waterproof condition.

Unfortunately, the fineness of machining processes in consumer grade electronics is insufficient to make a reliable surface to surface match between the hole and the conducting metal used to transmit the audio signal and the difference in thermal expansion between the two materials means that even a perfectly matched pair of hole and receiver will vary in size depending on the conditions.

To solve this, we will have to add another material that is both waterproof, malleable, and suitable for a range of temperatures like silicone rubber. Now that we have our components, we will probably have to bind them together in some way such that none of the parts will move an appreciable amount if the device is dropped, someone tries to insert an incorrect object into the hole, if the device is exposed to extreme conditions, if the device simply gets old and the silicone degrades, or any number of other possibilities.