The two speakers that you speak of I believe are separate from the headset speaker that is used for when the phone is placed to your head. I was looking at some tear down pictures from ifixit
but couldn't really make out anything.
It must be very cumbersome, but at least the phone works after being in the water.
Edit: This may sound weird, but back when I had a 4th gen iPod with a touch wheel, most people including me had problems where the headphones sounded like they were blown out and producing static. Not sure where I heard to do this, but putting the inside of the headphone barely on your lips and blowing in and sucking would make the speaker push in and out. Weirdly enough, this fixed the problem we had with the headphones.
Not saying that you should try this, but I wonder if the speaker needs to be flexed and pushed around a bit.