In electricity, there are two types of devices. Ohm's Law devices, and switch-type devices. (The things they never tell you in formal education make life so much more difficult for the uninitiated to understand).
In an Ohm's Law device, a resistor or buzzer for instance, voltage maps current. That is, the higher the voltage, the higher the current. This means that at zero current, there is zero voltage. At full current, there is full voltage. As current goes up, voltage goes up. Double the voltage across it, double the current through it. Or, double the current through it, double the voltage across it. Doesn't matter what else is happening in the circuit. (Assuming all other things with the Ohm's Law device being the same - temperature, for instance). If you know what happens to the current through a resistor, you know what happens to the voltage across it, and vice-versa. Nothing else in the circuit matters.
A switch-type device is exactly the opposite. They are NOT Ohm's Law devices, no matter what the 'experts' or 'professionals' or 'scientists' tell you. They just want to confuse you, and obfuscate reality. In a switch-type device, current is inversely proportional to voltage. An open switch has full applied voltage across it, but zero current. A closed switch has zero voltage across it, but full current. Some switch-type devices do not suddenly go from full open to full closed, they gradually close or open. Diodes, capacitors, transistors, coils (inductors) for example. In these devices, as the current goes up, the voltage goes down, or as the voltage goes up, the current goes down, somewhere between full open and full closed. The graph of voltage and current go in opposite directions and cross as the switch goes from open to close.
A capacitor is a switch-type device. The higher the voltage, the less current goes through it. A discharged capacitor is a closed switch - it has full current through it, but no voltage across it (a short). A fully charged capacitor is an open switch. It has full voltage across it, but zero current through it.
In your circuit, at rest (SW1 open) the capacitor is discharged. Therefore C1 is a closed switch. When S1 is closed, the voltage V1 is ground, or zero, volts. The capacitor is a short. A closed switch. Full current, no voltage. Thus, the voltage across the buzzer is zero (in parallel with a short, or closed switch). There is essentially one path through the circuit - from the battery through closed switch S1 through R1 through the closed switch C1 to ground. The buzzer is shorted out by C1. Since the buzzer is an Ohm's Law device, no voltage means no current through it. Thus, no sound. All the applied voltage falls across R1, and as an Ohm's Law device (the only one in the circuit at this stage) it has full voltage and full current.
As the capacitor charges, it changes from a closed switch to an open switch. As the voltage across it goes up, the current through it goes down. When it charges completely, it is a completely open switch. Full voltage, no current. Now, the buzzer is no longer shorted by the capacitor. The buzzer has full voltage across it. V2 is the 'charged capacitor' voltage, or the 'open switch' voltage of the capacitor. it will not quite be 9v. as the current through R1 to the buzzer in this new series circuit (C1 is essentially not in the circuit - it is an open switch) will cause a voltage drop across R1 (an Ohm's Law device). No current goes through the open switch capacitor, the current now bypasses this open switch and goes through the R1-Buzzer series circuit.
Ignore the quagmire theory they teach you in formal electricity classes about capacitors, coils, and such that they teach to make them LOOK like Ohm's Law devices. They just want to make it complicated so the average person can not understand it, thus making them look like 'experts'.
Categorize devices into Ohm's Law devices and switch-type deices, and so much starts to make sense without all of the hyperbole.
