Can I use a potentiometer to reduce sound level in a speaker?
That's a way, but it's better suited for eadphones, as the resistance will consume a power comparable to the speaker, so you need a fairly high power resistor.

The alternative is to use a power transistor, and then you would need only a circuit to bias it, and that can be generated also with a voltage divider, with the potentiometer.

The problem is, as Russel said, that the loudness is logarithmic with the power delivered, so you would need an exponential output from the transistor. I think that for achieving that you can use a MOSFET, that gives you an exponential transconductance (that is, for a linear increase of the input voltage, the current scales exponentially) but that's theory.