Can I use WTV020 sound module as SD card reader? I want to try this example, A Simple Function for Reading CSV Text Files, in order to read a .csv file from an SD card, but I don't have the SD card reader module and can't find it in my country. 

What I have found everywhere is this sound module, WTV020-SD, and I wonder if I can use that to read a single .csv file from it, using the SD Arduino library. I checked the module's documentation and there's nothing about it in there.
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.