I'm trying to control LED depends on the voice command being recognized. But our professor asked me the value of "buf" so that the value myVR.recognize(buf,50) will be the value of "ret".
You need to look at the documentation of the library that you did not provide a link to, so see what the recognize() method does. The return value from the function is stored in the variable named ret. The value will NEVER be "ret".