To exploit comparisons among classes of vertebrates and invertebrates, and between higher and lower levels of the brain, and between modalities, some important needs and opportunities for new research into the way central processing of acoustic input takes place are pointed out. Most of these are suggested by unfamiliar results on fish and reptiles that call for new controls in mammalian experiments as well as more systematic study of nonmammalian taxa. Three frameworks or basic agendas are outlined; i) systematic comparison of dynamical properties to acoustic variables including especially repetition at different rates and the related states of expectation; ii) comparison of response measures, including especially sequences such as oscillations and measures of assembly cooperativity such as synchrony, coherence and bicoherence; and iii) comparison of auditory subsystems, including especially modal categories such as complex feature selective regions and small sets. Some recent and some new results are summarized on human acoustic and non-mammalian event related potentials (ERPs) in response to expectations. When a regular and frequent standard stimulus is omitted, the omitted stimulus potential (OSP) after conditioning with low repetition rates (long ISIs--1-3 s) is a slow, broad positivity P600-900), previously known. With high rates (ISI < 1 s), a new form of response appears, with fast components (P22), different dynamics and less dependence on attention. Slow and fast OSPs each show a constant peak latency after the due-time of the missing stimulus, as though a temporal expectation has been learned. Unlike the visual OSP we have reported earlier, both fast and slow can occur together in the 1-2 Hz range. Very few conditioning stimuli suffice to create the "expectation" that causes an OSP--only two for the slow type. These and more familiar ERPs, considered in human subjects to index cognitive events, need to be compared in other classes of vertebrates and invertebrates.