What is the relationship between speaker volume change and resistance change?
Guess  you have a source, a resistance (assume Ri is your poti) and a load resistor

So let us assume the load is variable (multiples of Ri) and the Ri is const, let us plot voltage and current at the load [voltage blue, current red]

let us also plot Power and efficiency factor at the load [Power blue, efficiency factor red]

we can see the following

max. Power at load is at 50% efficiency
Power and efficiency cross at 75% when Load is 3 times Ri
when the ratio RL/Ri is big (&gt;1), the efficiency is good


Edit: given the load is a speaker, volume of the speaker is proportional to the voltage