I have exactly the same problem, 10 - 15 minutes after a brief bit of radio in the bathroom, but the phone wasn't inside the actual shower, I plugged my noise cancelling headphones in and immediately heard a couple of small pops. Thought nothing of it to be honest, but as my journey to work went on the noises got louder and more distorted and seemed to be being caused by the background noise itself. Then the phone just went mental, started getting very hot and taking me to the Smart Connect App for no reason, the speakers stopped working completely, could call people, but not hear anything (dial tone or voice), even after multiple re-starts nothing. Tried calling my phone from my desk phone and the speakers were working for ringtone/notification, but not music and voice! Then after turning it off and leaving it for a while, things seemed to come back to normal, but then I tried plugging the headphones in and once again the phone went mental. Running stock ROM, unrooted on 17.1.A.2.55 (STILL!) I'm a bit miffed to say the least. The phone has in some respects exceeded my expectations, but sometimes feels like a half-baked product.
Phone: Sony Xperia Z2
Current ROM: Stock
Previous Phones: HTC One X.. HTC Desire.. Nokia E71.. Nokia N73.. Nokia 6680..