The speech-to-song illusion is a perceptual effect emerging at the interplay of two cognitive domains, music and language. It arises upon repetitions of a spoken phrase that shifts to being perceived as song, and varies in the likelihood, ease, and vividness of its occurrence among individuals. A prevailing explanation of the illusion suggests that listeners’ attention shifts to rhythm and melody of the phrase once their involvement with the linguistic meaning subsides. The present study tested this mechanism by manipulating meaning plausibility and structural complexity of French and English phrases and by obtaining measures of attentional and working memory capacity from 80 French and English listeners who were exposed to repetitions of sentences in their native language. The results show that the transformation was facilitated in listeners with fewer cognitive resources and in less plausible, more complex phrases, which is at odds with the previously proposed mechanism underpinning the speech-to-song illusion. The illusion-promoting effect of musical training was visible only in simple but not in complex phrases. We propose a new account of the perceptual transformation from speech to song as a cognitive effect arising from the accumulation of music-related priors in a linguistically ambiguous context of massed repetitions.