Maybe no water damage but the fall might have either damaged the speaker grill or most likely, moved the speaker from it's original position. So while the speaker might still work, it's now making sounds against a surface it's not supposed to, and that might be the reason this sounds muffled and/or distorted. I had that happen to me once on a Galaxy S2. That time they replaced it under warranty since it was only after two days and there were no drop marks on the phone