Listeners usually have no difficulties telling the difference between speech and song. Yet when a spoken phrase is repeated several times, they often report a perceptual transformation that turns speech into song. There is a great deal of variability in the perception of the speech-to-song illusion (STS). It may result partly from linguistic properties of spoken phrases and be partly due to the individual processing difference of listeners exposed to STS. To date, existing evidence is insufficient to predict who is most likely to experience the transformation, and which sentences may be more conducive to the transformation once spoken repeatedly. The present study investigates these questions with French and English listeners, testing the hypothesis that the transformation is achieved by means of functional re-evaluation of phrasal prosody during repetition. Such prosodic re-analysis places demands on the phonological structure of sentences and language proficiency of listeners. Two experiments show that STS is facilitated in high-sonority sentences and in listeners’ non-native languages and support the hypothesis that STS involves a switch between musical and linguistic perception modes.