input raw audio info:
sample rate: 44.1k
sample format: 16bit signed integer, little endian
channel numbers: 2
encoding the raw pcm audio into .wav using ffmpeg, command line:
ffmpeg -f s16le -ar 44.1k -ac 2 -i input_file.pcm output_file.wav
thanks to http://stackoverflow.com/questions/11986279/can-ffmpeg-convert-audio-from-raw-pcm-to-wav