You don't make a lowpass filter by connecting a capacitor in parallel with a speaker. The capacitor would just be a short circuit to the amplifier for high frequencies and destroy the amplifier.
You take the music source from the radio or CD player, connect an adjustable resistor to it then it feeds an amplifier's input and feeds a capacitor to ground. The adjustable resistor feeding the capacitor to ground is a single RC passive lowpass filter. It passes most of the deep bass but not all of it, a lot of the upper bass, some of the mid-range and a little of the high frequencies. Because it is only a single stage it makes a very poor filter.
A pretty good active filter has 3 or 4 RC stages and has feedback. It has a flat response to all bass frequencies then sharply cuts-off higher frequencies.