Is it possible to use two or more WAVE SHIELDS to an Arduino to play two or more SOUNDS at a time?
No not straight out of the box. The way the wave shield works is that the SD card is connected to the SPI bus and the D/A is connected to some pins and accessed by bit banging. You would have to hack the libraries, and change the pins used by the D/A and SPI enable line on the other shield.
Having done that I am not sure you would be able to get a good sample rate. The wave shield can only support 22KHz sampling so that may be halved.
Best use a Due for this project as it has two built in D/As