Quoted from newmantjn:I don't have a great suggestion resistor size. If you put it across the lugs, bigger = louder. If you put it in series, bigger = softer. You could try - Just perusing a Ship Ahoy schematic, I see that they use a 75ohm resistor in series to step down to 6v for a light, if that helps any.
Put in a variable resistor (potentiometer) in series. Then you can vary the strength of the coil to give the volume you want.