I decided to open the cover to see what happen inside the radio.
The problem solved after I disconnect and reconnect the board. (see the picture) .
1. Use small (plus +) screwdriver to remove 5 screws.
2. After opening the cover,
3. Remove one screw at backside.
4. Disconnect and connect the board. Be carefully, you must put the connector back properly.
After disconnect and reconnect the board, my radio volume is back to normal again.