Guys, I have that module WTV020m01.. I tried to turn it on with push buttons instead of arduino. It couldn't play .ad4 files, nor wav. I tried everything, lowering down the records volume and so on.. I tried to connect the 6-th SD CARD pin with 8-th of the module, it didn't work also. My SD card is 2GB, I dunno what is the manufacture, but THE SOLUTION WAS FORMATING THE SD-CARD INTO FAT(DEFAULT) FORMAT SYSTEM. When the module didn't work, the sd-card was in fat32 system, but after formating the card again in just FAT i worked perfectly.
I hope that works for most of you too.