Water damage on an iPhone 6s - Sound not working (Locked into Headphone mode)
I had the exact same thing happen to me. Got water spilled on my iPhone 6s. I immediately wiped it with a tissue paper and let it dry.

Immediately afterwards I noticed that while the phone rang fine for incoming calls, other sound (media) didn't play through the speakers and the microphone stopped working. The iPhone was basically locked into headphone mode (noticed via the volume change HUD), where and sound was not playing back through the speakers.

I was able to revive it by doing the following two things:


Blowing compressed air into the headphone jack.
Repeatedly plugging/unplugging the 3.5mm pin of the Apple EarPods in the headphone jack.


After a while, I was able to get the iPhone out of the headphone mode (I think primarily due to the second approach) and the speakers and microphone started working alright.

However, in my case, the Taptic engine took the toll of water damage and it stopped working after a while (I was never able to get it to work).

P.S.: When it happened to me, I search the Web and found a lot of posts where people have narrated the same issue (making me believe that its particular with iPhone 6s). For most cases, plugging/unplugging the EarPods/headphone resolved the issue.

P.P.S.: Here's a Apple Support document which discusses water and other liquid damages to iOS devices and the built-in indicators.


Water and other liquid damage to iPhone or iPod isn't covered by warranty



  Phone and most iPod devices are equipped with Liquid Contact Indicators (LCIs) that you can see from the outside.
  
  An LCI will activate when it contacts water or a liquid containing water. The indicator's color is normally white or silver, but when it contacts water or a liquid containing water, it will turn fully red. An LCI won't activate because of humidity and temperature changes that are within the product's environmental requirements.
  
  To find out if the LCI in your device contacted liquid, find your product in the table (in the linked document) and locate the LCI or LCIs in your device. To help you see the LCI, use a lighted magnifying glass and angle the light or the device until you can see the LCI.