Pulse Density Modulation (PDM) and its subcategory, Pulse Width Modulation (PWM), are pretty good ways to make a Digital to Analog Converter (DAC) considering it only uses one GPIO pin compared to the many needed for other methods.

This is a PDM test code that I wrote for my personal project. I've taken care to carefully comment it, so I think it should be easy enough for someone else to convert into a format suitable for their own project.

Best of luck, and hit me with any questions you've got!