Can I use WTV020 sound module as SD card reader?
It seems unlikely, for if you compare the schematic of the WTV020-SD (card reader part only) from the data sheet that you provided:



to the schematic of a bog standard SD card reader:



they are not the same. Instead of interfacing directly to the Arduino, as the standard SD card reader does, the WTV020-SD has additional logic, in the form of a WTV020SD-20S IC.

Therefore the code on the Arduino forums is highly unlikely to work.