Contrary to the classical view, recent neuroimaging studies claim that phonological processing, as part of auditory speech perception, is subserved by both the left and right temporal lobes and not the left temporal lobe alone. This study seeks to explore whether there are variations in the lateralization of response to verbal and nonverbal sounds by varying spectral complexity of those sounds. White noise was gradually transformed into either speech or music sounds using a "sound morphing" procedure. The stimuli were presented in an event-related design and the evoked brain responses were measured using fMRI. The results demonstrated that the left temporal lobe was predominantly sensitive to gradual manipulation of the speech sounds while the right temporal lobe responded to all sounds and manipulations. This effect was especially pronounced within the middle region of the left superior temporal sulcus (mid-STS). This area could be further subdivided into a more posterior area, which showed a linear response to the manipulation of speech sounds, and an anteriorly adjacent area which showed the strongest interaction between the speech and music sound manipulations. Such a differential and selective response was not seen in other brain areas and not when the sound "morphed" into a music stimulus. This gives further experimental evidence for the assumption of a posterior-anterior processing stream in the left temporal lobe. In addition, the present findings support the notion that the left mid STS area is more sensitive to speech signals compared to the right homologue.