If you have bought the DX Shodophone (normally the Bandai Asia version) that have the soft sound volume issue, and you really want to increase the ridiculously soft sound volume, you might want to try the following AT YOUR OWN RISK:
(1) Open up the DX Shodophone and get to the circuit board.
(2) You will see a SMD resistor R4, which is recognizable as a little small black piece with "R4" printed beside it.
(3) Connect (solder) a wire as shown in picture below. This is to bypass R4 and let current (signal) from the processor (sound chip) to flow straight into the speaker.
I tried to change the speaker initially, after reading from some website that this will work, but it seemed to fail... Then realised there is a resistor before the speaker path, so decide to try... It works!!! My DX Shodophone now is having a sound volume similar to that of the typical DX toys... Nice... Maybe a bit too loud... Will do the mod to my little nephew's DX Shodophone, which he had brought home to play earlier...