This was a very rough Proof of Concept I made back in 2018 and recently came across the code for. It is a volume mixer for Windows that lets you control the volume of individual applications on a PC.
It uses an Arduino Pro Micro, a custom PCB, and four potentiometers. The Pro Micro communicates with the C# application over a USB serial connection. The pots are connected to A0-A3 on the Pro Micro.
Included are the 3D model files that I made for the case, the gerber files for the PCB, and all the code that I could find.
All code was written on Visual Studio Community 2017 and has not been tested on any newer versions.