Apart from detecting sounds, vertebrate ears occasionally produce sounds. These spontaneous otoacoustic emissions are the most compelling evidence for the existence of the cochlear amplifier, an active force-generating process within the cochlea that resides in the motility of the hair cells. Insects have neither a cochlea nor hair cells, yet recent studies demonstrate that an active process that is equivalent to the cochlear amplifier occurs in at least some insect ears; like hair cells, the chordotonal sensory neurons that mediate hearing in Drosophila actively generate forces that augment the minute vibrations they transduce. This neuron-based force-generation, its impact on the ear's macroscopic performance, and the underlying molecular mechanism are the topics of this article, which summarizes some of the recent findings on how the Drosophila organ of hearing works. Functional parallels with vertebrate auditory systems are described that recommend the fly for the study of fundamental processes in hearing.