Sound envelope cues play a crucial role for the recognition and discrimination of communication signals in diverse taxa, such as vertebrates and arthropods. Using a classification based on metric similarities of spike trains we investigate how well amplitude modulations of sound signals can be distinguished at three levels of the locusts auditory pathway, receptors, local and ascending neurons. The spike train metric has the advantage to provide information about the necessary evaluation time window and about the optimal temporal resolution of processing, thereby yielding clues to possible coding principles. It further allows to disentangle the respective contributions of spike count and spike timing to the fidelity of discrimination. These results are compared to the traditional paradigm using modulation transfer functions. Spike trains of receptors and two primary-like local interneurons enable an excellent discrimination of different AM frequencies, up to ~150 Hz. In these neurons discriminability depends almost completely upon the timing of spikes, which must be evaluated with a temporal resolution of <5 ms. Even short spike train segments of 150 ms, equivalent to 5 to 8 spikes, suffice for a high (70%) discrimination performance. For the third level of processing, the ascending interneurons, the overall discrimination accuracy is reduced. Spike count differences become more important for the discrimination while the exact timing of spikes contributes less. This shift in temporal resolution does not primarily depend on the investigated stimulus space. Rather it appears to reflect a transformation of how amplitude modulations are represented at more central stages of processing.