Arbitrary communication systems can emerge from iconic beginnings through processes of conventionalisation via interaction. This paper explores whether this process of conventionalisation occurs with continuous, auditory signals. It conducted an artificial signalling experiment. Participants either created signals for themselves, or for a partner in a communication game. It found no evidence that the speech-like signals in the experiment became less iconic or simpler through interaction. The paper hypothesises that the reason for the results is that when it is difficult to be iconic initially because of the constraints of the modality, then iconicity needs to emerge to enable grounding before conventionalisation can occur. Further, pressures for discrimination, caused by the expanding meaning space in the study, may cause more complexity to emerge, again as a result of the restrictive signalling modality. The findings have possible implications for the processes of conventionalisation possible in signed and spoken languages, as the spoken modality is more restrictive than the manual modality.