I made this page to try to help a few people correct a few of their misconceptions.
The output voltage of an amplifer operated from a single supply when undriven will be at half the supply voltage. This d.c.is isolated from the loudspeaker by placing a capacitor in series.
At switch on, the capacitor charges up, through the loudspeaker, to half the supply value. There will, therefore, be half the supply voltage across the capacitor and zero volts across the 'speaker.
When the amplifier is driven, the voltage at its output changes with the signal, so the standing d.c. is modulated by it. It is the sum of a d.c. and an a.c.
The a.c. component passes through the capacitor and is applied across the 'speaker. Since the voltage on the driven side of the 'speaker is normally zero, it will now swing between a positive and negative values.
The following diagram shows this.
The output voltage of an amplifer operated from a dual supply when undriven will be at zero volts. There is no d.c. so no capacitor is necesssary.. When the amplifer is driven, its output changes, swinging between positive and negative values. This a.c. signal is applied directly across the 'speaker, and is shown in the next diagram.