Capacitor question please help

I have a simple question, is the charge of the capacitor constant? How can I deduce this?

No, it's not constant. The charge on the capacitor changes as a result of the plate distance varying in response to air pressure changes.

As the capacitance varies, the current flowing in the resistor, continually responds either to allow additional charge supplied by the 3V DC source, or, dissipates excess energy as heat when charge flows in the opposite direction.

In this way, an ac induced potential difference is developed across the resistor as a facsimile of the air pressure changes (sound) driving the plate distance.

The equations modelling behaviour are:

$Q = \frac{C}{V}$

and

$C = \epsilon_{o} \epsilon_{r} \frac{A}{d}$

great! Thank you so much!!

Oooops edit: Q = CV !