Brain-computer interfaces (BCIs) are beneficial for patients who are suffering from motor disabilities because it offers them a way of creative expression, which improves mental well-being. BCIs aim to establish a direct communication medium between the brain and the computer. Therefore, unlike conventional musical interfaces, it does not require muscular power. This paper explores the potential of building sound synthesisers with BCIs that are based on steady-state visually evoked potential (SSVEP). It investigates novel ways to enable patients with motor disabilities to express themselves. It presents a new concept called sonic expression, that is to express oneself purely by the synthesis of sound. It introduces new layouts and designs for BCI-based sound synthesisers and the limitations of these interfaces are discussed. An evaluation of different sound synthesis techniques is conducted to find an appropriate one for such systems. Synthesis techniques are evaluated and compared based on a framework governed by sonic expression.