#!/bin/bash

if [ $# -eq 1 ] ; then

    if expr "$1" : "^.*\.....$" ; then
        ffmpeg -i "$1" "${1:0:-4}mp3"
    fi
    if expr "$1" : "^.*\....$"  ; then
        ffmpeg -i "$1"  "${1:0:-3}mp3"
    fi
fi