as mark said in a pcm stream left and right is always next to each other. this means:
left|right|left|right
how big these blocks are is stored in the waveformat. just take a look at bitspersample
that means if you have a 16bit stream each block is 2 byte long
left|right|left|right
2 2 2 2 - byte
if you have a 24 bit stream you have to take 3 bytes... should be everything clear now :)