And what would that do? Haha
lower the volume
in the older analog audio systems the volume knob is a potentiometer (like a guitar's volume knob for example), which is a variable resistor. adding just a resistor will set the volume lower to a specific level, exactly as if you turned the pot there and left it forever.
people make audio resistors, but they're for loudspeaker systems and thats overkill. so, no need to worry with those, if you find them in your googling....
edit: ie, an audio signal is an electric signal, so with a resistor you have the audible effect of lowered volume