The task of assigning concurrent sounds to different auditory objects is known to depend on temporal and spectral cues. When tones of high and low frequencies are presented in alternation, they can be perceived as a single, integrated melody, or as two parallel, segregated melodic lines, according to the presentation rate and frequency distance between the sounds. At an intermediate distance, in the 'ambiguous' range, both percepts are possible. We conducted an electrophysiological experiment to determine whether an ambiguous sound organization could be modulated toward an integrated or segregated percept by the synchronous presentation of visual cues. Two sets of sounds (one high frequency and one low frequency) were interleaved. To promote integration or segregation, visual stimuli were synchronized to either the within-set frequency pattern or to the across-set intensity pattern. Elicitation of the mismatch negativity (MMN) component of event-related brain potentials was used to index the segregated organization, when no task was performed with the sounds. MMN was elicited only when the visual pattern promoted the segregation of the sounds. The results demonstrate cross-modal effects on auditory object perception in that sound ambiguity was resolved by synchronous presentation of visual stimuli, which promoted either an integrated or segregated perception of the sounds.