Example circuit of fading tone?
My simple way I did what you want was I bought a really cheap small battery power "star trek sounds" toy from a gift shop for around $1 whose sounds were like I wanted, opened it up and took out the tiny circuit board in it and connected one of the three switch wires on the pcb to the output of my circuit with a 1k resistor and connected the voltage and ground lines to a 3.3volt wire and ground from my arduino. Each switch wire on the pcb makes a different sound and I easily found the one that made the sound I wanted.

So I got the sound done quickly and could spend time on function I wanted to make on the arduino instead of spending time in doing the sound part circuit and all. I know this is not a technical great answer like some are on this site but I am self taught so sorry.