@rewetiperi you got to open them up. Use these guides [[Topic:Sony WH-CH720N]] to work on your headphones. Remove both speakers and clean the circuit boards with something like +90% isopropyl alcohol. Replace the battery and wait for the speakers (drivers) to dry. For any questions about the board, wiring etc, post some well-focused pictures with your Question. That way we can see what you see and hopefully we can assist you fixing your headphones.
After that, reassemble and listen to what they sound like. It is entirely possible that you may have to replace the speakers (drivers) since they may have gotten shorted out. Water amge is one of the hardest to work with. It's hard to diagnose because multiple circuit may have shortened out. Give it a try and let us know how it works out.