#!/bin/bash
IN="$1"
[ -z "$IN" ] && echo "usage: $0 <in.flv> [<out>]" && exit 1

OUT="$2"
EXT=wav
[ -z "$OUT" ] && OUT=`basename $IN .flv`.$EXT

ffmpeg -i "$IN" -vn -acodec pcm_s16le "$OUT"