On arrival in the circuit area at Wedderburn, the pilot was unable to extend the landing gear by the normal method. The failure of the gear to extend was accompanied by a very loud high pitched noise in the headset and speaker systems. His attempts to lower the landing gear by the manual emergency system were also unsuccessful. As a result, the pilot elected to return to Bankstown. However, on arrival in the circuit, he was unable to communicate by radio to the Tower or other aircraft. Air traffic control staff recognised the pilot was experiencing difficulties with the landing gear and called out the emergency services. When the services were in place the aerodrome was closed and the pilot advised by signal light that he was cleared to land. The aircraft subsequently landed on runway 11 with the landing gear retracted. An investigation revealed a fault in the alternator field circuit which prevented the battery from being charged. The landing gear failed to extend by the normal means due to low battery voltage. The pilot was unable to lower the gear by the manual system because he could not gain access to the emergency gear extension handle due to an incorrectly fitted piece of interior trim. It was not determined how the trim came to be incorrectly fitted.