If you changed battery yourself, you've lost the tiny spring without noticing it. Carefully check around the place where you changed the battery. The spring itself is very, very tiny, so, be as concentrated as much as you can. That spring (coilspring named by watchmakers) transmits the signal from watch movement to the watch back cover where the sound membrane is located. In case if you find it, open the watch and insert that spring with the bent out end first in to the desired hole in watch movement (should be named COIL SPRING). After that re-assemble watch and the sound should work.
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If you changed battery yourself, you've lost the tiny spring without noticing it. Carefully check around the place where you changed the battery. The spring itself is very, very tiny, so, be as concentrated as much as you can. That spring (coilspring named by watchmakers) transmits the signal from watch movement to the watch back cover where the sound membrane is located. In case if you find it, open the watch and insert that spring with the bent out end first in to the desired hole in watch movement (should be named COIL SPRING). After that re-assemble watch and the sound should work.