There's a few different ways to add sound, from the high end it would be an arduino build, down to a $5 toy blaster, both work well it just depends how screen like you want it to sound. For lights the arduino allows quite a nice bank of lights, also a graph for ammunition level, at the cheap and easy end you can just add a high bright LED, one side comes from the battery the other goes to the switch.
A nice trick it to fit an output jack to your speaker so you can run a cable to your amp, really pumps out the sound