This article is from the sci.lang FAQ, by Michael Covington (mcovingt@ai.uga.edu) and Mark Rosenfelder (markrose@zompist.com) with numerous contributions by others.
[--markrose]
It varies. Tones are basically ignored in Mandarin Chinese songs,
for instance. (Does this make them hard to understand? Often, yes.)
However, Cantonese songs are generally written in such a way as to
preserve the relative pitch of successive syllables. E.g. a low tone
following a high tone will be on a lower note.
For more, see: http://deall.ohio-state.edu/chan.9/articles/bls13.htm
 
Continue to: