create a raw file with silence. for example 1 second,
16 bit
mono at 44100 Hz:
dd if=/dev/zero of=silence.raw bs=44100 count=2
and convert it to wav:
sox -t raw -r 44100 -s -w -c 1 silence.wav
sorry, that should be
sox -t raw -r 44100 -s -w -c 1 silence.raw silence.wav