okay, so this is about Cantonese and not Mandarin but perhaps it is applicable? The title of this paper is “How can the lyrics of a song in a tone language be understood? ”
ABSTRACT:
In a tone language, pitch variations are used to contrast word meaning. For example, the Cantonese syllable /si/ means ‘teacher’ when spoken in a high pitch and ‘yes’ when spoken in a low pitch. How is fundamental frequency (F0) used to signal lexical tones that occur in songs? In an examination of Cantonese songs, it was found that songwriters abandon the ratio scale of F0 differences that is applied to lexical tones in carefully read speech and instead use an ordinal scale. For example, a high tone that is normally 12% higher than a mid tone in speech can be realized as any higher F0 (but never a lower F0) in songs. A perceptual experiment showed that native Cantonese-speaking listeners similarly apply an ordinal F0 scale to arrive at the lexical meaning of the lyric. This ratio-to-ordinal mapping in Cantonese songs ensures the musicality of the melody while preserving adequate identifiability of lexical tones in the lyric.
available at
http://72.14.253.104/search?q=cache:GJ0CwAymFQMJ:www.communication.northwestern.edu/publication/173803fe932c6e3b93.doc+mandarin+tone+song&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a