A wireless microphone (1) comprises a radio transmitter for transmitting radio signals carrying audio signals to a hearing device. The wireless microphone (1) further comprises at least one coupling element (11) for detachably fixing one of various interchangeable functional modules (2) to the wireless microphone (1). The coupling element (11) is designed to fix the interchangeable functional module (2) to the wireless microphone (1) by mating with a coupling element (21) of the interchangeable functional module (2). The coupling elements (11, 21) comprise electrical contacts (12, 22) for electrically connecting the interchangeable functional module (2) to the wireless microphone (1) when the interchangeable functional module (2) is fixed to the wireless microphone (1) by means of said coupling elements (11, 21). The coupling element (11) constitutes a bidirectional electrical interface between the wireless microphone (1) and the interchangeable functional module (2). The wireless microphone (1) can be limited to an assistive listening device with basic functions and features, extendible by the user with extra functions and features as needed.