The amplifier is the last stop before the signal reaches the loudspeakers. Power amplifiers boost, or amplify, electronic audio signals sufficiently to move the speakers. They do this by increasing the gain (the voltage and power) of the signal from line level to speaker level.
Line level is around one volt. Speaker level depends on the type of system. It can range from:
4 to 20 volts or more for stereo sound systems
up to 70 volts in a distributed sound system
up to hundreds of volts in large venues...