Always best to only learn things at the time they're needed. And the great thing is, a volume slider is something you can roll out for every game so not a time sink either.
If you're in Godot, then check out the HSlider control node and AudioServer (specifically the set_bus_volume_db method). In other engines, you just need a slider control that then interacts with the audio processing to set the volume based on the value of the control.
Best of luck and maybe we'll see you in another jam some time!