The Story of Chinese Character :

云 depicts a cloud. Also, when somebody speaks, water vapor is released from his mouth, makes artificial ’cloud’, so 云 also means speaking. However, after the appearance of 雲，云 is seldom used to mean cloud. In simplified Chinese, there is no 雲，云 is used to represent both meanings above. On the other hand, just for fun, a cloud is similar to the dialogue bubble in comic, that is why 云 means speaking.