A modal analysis of flow-acoustic wave propagation through slit array metamaterials is presented. It is demonstrated that the transmission-coefficient change versus flow speed is a sensitive function of frequency. Our results further confirm that transmission resonance positions and resonance widths change significantly with the flow speed. As a reverse application, the present metamaterial slit structures allow for flow tuning of slit cavity modes, design of surface bound states such as superlens applications where a broad frequency operation interval is sought. Finally, it is shown rather surprisingly that the flow-acoustic coupling is almost independent of the angle of incidence.