Let's compare it with shell bone script for character Say,
We see there are two more short lines in character Sound, one on the top, and another in the Mouth (口） at the bottom. Both are pointers to tell where the voice is created. The top line is pointing the tip of the tongue, and the short line inside the mouth is telling that the sound is from the Mouth.
The small seal script for Sound is
Let's compare it with small seal script for character Say,
same as the above, the character Sound has two more short lines, to emphasize where is the sound from.
The clerical script for Sound is
The top part changed to 立，which means Stand Up, nothing to do with voice, or tongue. This change should be caused by simplified writing, which omitted some strokes; or miswriting.
The current standard printing script is
almost same as the clerical script.
I need to point out that the character also means Voice sometimes. I will discuss the difference more in post for Character Voice.
Its Pinyin is Yin1.