Low quality simple audio mixing?


I want to crudely mix a tone from a micro controller into audio from a video camera. The tone will be a square wave on a gpio at 5v. I would like to set the volume of the tone but maintain the original audio with minimal volume loss. The quality of sound is very low as it is for adding a variometer to FPV on a fixed wing RC plane. So the sounds will be mainly wind and motor noise. I am hoping a simple resistor network will do the job but not sure where to start.

Thanks William