Ohh, that must be because of the waer, try submerging that part of the phone in rice, and leave it that way for about an hour or two, it might go back to normal then.
If it was in water then do as Zahi_znaush. You can also try my way, vacuum the speakers with the lowest power. Water drops should go out of the speaker.
Please be wary of vacuuming the speakers, this can lead to damage of the speaker due to the force/static this can generate. Normal heat generated by usage should be enough to dry out the speaker in a short period fo time.
Personally, I have done all the testing, but, but nothing resolved, this only occurs in speakerphone function when I hear music, game play, the two high falente works perfectly.
