Atrial fibrillation is irregular activity that occurs in the atria that causes an ineffective ejection of blood into the ventricles of the heart. A heart murmur is an abnormal sound that can be heard when listening to the heart. A murmur may be heard with atrial fibrillation, but is not always heard with one.
Atrial fibrillation is essentially a "quivering" type of heart beat. A heart murmur is just that. A sound. Both can have several causes. You can have one without the other. And I could go on but I don't think I should unless you ask me to.