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Speaker normalization is essential for speech perception, particularly in tonal languages like Cantonese. Listeners can normalize talker variability under cognitive load, yet the neural mechanisms remain unclear. We recorded EEG from Cantonese speakers during a tone normalization task with and without concurrent visual tasks. Although behavioral performance was unaffected by visual distraction, neural data revealed distinct processing strategies. Without load, alpha-band desynchronization emerged, but load conditions triggered sustained parieto-occipital alpha activation, reflecting active suppression of visual distractors. High load uniquely induced prefrontal delta-band inhibition, indicating an attentional shift from internal visual processing to external auditory stimuli. These results suggest that normalization under load is not automatic. Instead, the brain actively reallocates resources to shield speech processing from distraction, supporting an active control hypothesis.