Object-oriented programming (OOP) has been for many years now one of the most important programming paradigms used in a variety of applications. Digital audio signal processing can benefit largely from this approach for systems development. In this paper a number of approaches to using object-orientation in audio processing systems are reviewed. Existing systems of audio processing are introduced and discussed in detail. The paper also draws attention to the different OOP techniques enabled and supported by these systems. Comparative code and tutorial examples are included, providing an insight into the development of signal processing applications using objects.