It's a $5 adapter/cable on Amazon and can delivered within a few hours if in US. https://www.amazon.com/Connector-Female-Amplifier-Speakers-Theater/dp/B07DNXZBW4

At the 19:50 mark of this video https://youtu.be/iFuuJMB9_c8?t=1190 I show how to do a simple volume knob. This can be purchased from here: https://www.amazon.com/gp/product/B01MS6YXLR All you need to do, is get 2 RCA to 3.5mm adapters. 1 would be like these https://www.amazon.com/gp/product/B06XC5KWJN that would lead to the audio out of the PCB/Pi/Pc, and then an RCA to 3.5mm female like these: https://www.amazon.com/dp/B07DNXZBW4 which would of course plug into the speaker cable. And VIOLA, easy volume control with no soldering or clipping.