A speaker is an electro-mechanical transducer; it converts an electrical representation of music/voice to sound; typically a paper cone attached to a coil of wire wrapped around a cylinder. This coil is suspended in a magnetic field. Current provided by an amplifier flows back and forth through the coil and this deflects the coil, (and the attached paper cone), back and forth in the magnetic field in response to the direction and magnitude of the current flowing through the coil; this in turn moves the air creating sound which corresponds to the signal current from the amplifier.
There may be several speakers in a speaker cabinet; each specifically designed to accurately reproduce a particular range of frequencies. There is usually an electronic circuit (a cross-over network) inside the cabinet to sort the frequency bands from the composite signal and deliver each band to the particular speaker designed to reproduce that range of frequencies.