A basic capacitor works as two parallel plates carrying opposite charges and have an electrostatic force holding the charges in place (without making any contact); can't an electrostatic loudspeaker be considered a capacitor since it works by having a charged diaphragm between two plates of opposite charge?
Yes, an electrostatic speaker is a capacitor. From the Wikipedia article "Electrostatic loudspeaker":
The electrostatic construction is in effect a capacitor, and current is only needed to charge the capacitance created by the diaphragm and the stator plates
There are, as you might expect, microphones that are essentially capacitors too.