Increasing evidence underlines the functional importance of non-phase-locked cortical oscillatory rhythms. Among the different oscillations, alpha (8–12 Hz) has been shown to be modulated by anticipation or attention, suggesting a top-down influence. However, most studies to date have been conducted in the visual modality and the extent to which this notion also applies to the auditory cortex is unclear. It is furthermore often difficult to dissociate bottom-up from top-down contributions in cases of different stimuli (e.g., standards vs. deviants) or stimuli that are preceded by different cues. This study addresses these issues by investigating neuronal responses associated with intrinsically fluctuating perceptions of an invariant sound. Sixteen participants performed a pseudo-frequency-discrimination task in which a “high-pitch” tone was followed by an aversive noise, while the “low-pitch” tone was followed by silence. The participants had to decide which tone was presented even though the stimulus was actually kept constant while pseudo-randomized feedback was given. EEG data show that auditory cortical alpha power decreased by 20% in “high-pitch” trials relative to trials in which a “low pitch” was perceived. This study shows that expectancy of aversive feedback modulates perception of sounds and these fluctuating perceptions become manifest in modulations of sound-related alpha desynchronizations. Our findings extend recent evidence in the visual and somatosensory domain that alpha oscillations represent the excitatory/inhibitory balance of sensory cortical cell assemblies, which can be tuned in a top-down manner.