prosody...
First of all, prosody is a way of looking at the rythmic structure of language, it is not necessarily that structure itself. Even if something lacks rhythm, and that's pretty hard to imagine, even in something like a chant, it has a prosody.
If a line is able to be sung quickly and in little space, it is the skillful assembling of syllables that makes it possible. There is a natural prosody that English tends to follow, and without it we'd all sort of sound like the dwarf and Laura Palmer in the red room on Twin Peaks.
All poems and speech in general, have metre, and it is that that gives them their music. The rhythms are more complex when you add notes and percussion to the mix. Whether or not you are conscious of words' rhythm when you are writing is another question. It seems inevitable that you must be, however, if words and music are to have anything to do with one another.