I think lots of people have already done this. It doesn't seem like a difficult project though your requirements, whatever they are, may limit your choices. I googled on "ipod audio amplifier schematic" and found quite a bit of stuff - some that might get close to what you need. Tell us what's wrong with what you find - as a way of helping describe what would suit your needs. Keep in mind that a 9 volt battery is only going to last so long - and louder means less battery life.