Can an electrostatic loudspeaker be considered as a capacitor?
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.