....What ...in the... are you talking about?
The only reason why electrostatic planar headphones are called that is because they operate by having a 'static' electric charge on the membranes, sandwiched between perforated metal plates that act as electrodes generating an electric field onto the charged membrane - audio signals vary the field, so the membrane vibrates and makes sound
like any other audio transducer, by pushing air. Guess what the perforations are for in the electrodes? They are for the air to move through in order to conduct sound to your ears. This ain't rocket science.
Having the membranes lighter than air or not has nothing to do with anything; thinner membranes just ensure better transients, at an expense of more difficult tensioning in the manufacturing process, and often more difficulty in pushing air for low frequencies. There is no "electrostatic effect" going on, the electric fields generated by the electrodes in electrostatic headphones aren't making any sound.