A key skill for social robots is the ability to communicate their inner state to humans. In this paper, we explore abstracted robot-specific ways of interaction as an alternative to human-like or animal-like social cues. In particular, we present a sound system as a novel modality that extends a robot's ability for non-verbal communication. Unlike prior work which used pre-recorded audio samples to this end, we propose a flexible architecture with a generalized sound synthesizer that uses the principle of modulation to shape the sound in real-time by external and internal stimuli from the robot or the interaction. This allows for almost unlimited possibilities in the design of an expressive auditory social cue for human-robot interaction. We instantiate the architecture and report on example design choices for the sound synthesis principle, the real-time synthesizer, the sound modulation routings, and a sound sequence composer. We then demonstrate the system's ability for affect communication of primary and secondary emotions on a social robot.