That's why i never wet my phone.
I saw quite a lot of bad review about the waterproof since xperia z.
Not to said Sony's quality is bad, but i really not so confident about the waterproof function.
I saw quite a lot peoples lost their screen in XDA forum. If your phone is only the speaker problem, i guess it is quite lucky :/

BTW, i would advise you, go to tell the SC, you already hard reset the phone before, the other SC also did the troubleshooting too. There is nothing more can check, unless change the speaker. Insist and tell them skip the unnecessary diagnosis, just change the speaker for you.