Found the manual but nothing about the sound.
If I recall sometimes the part that makes the sound can be damaged when replacing the battery, open it up and check the (little wire type vibrating bar) sound bar and adjust if necessary.
http://www.casiodigitalwatches.com/manuals/0809.pdf
The battery that came out of it will have a number/voltage on it... just replace it with same..
If this isn't possible, take the watch to Batteries.bulbs, etc,, they can fit it with one.