One of the reasons that can cause this is dirt, debris, or other grime stuck in the speaker or speaker housing. Gentle use a cloth and wipe away any on the outside or gently blow or used compressed air very lightly go over the speaker. This will help clear any grime out of the speaker and then test the sound again.
One of the ways a lot of people remedy getting water on a device is to fill a bowl with rice. Submerge your device in the dry rice and let it sit for a few days. The rice will draw the moisture out of the device and hopefully there will be no harmful side effects. This is the first step I would try.