Is there one main melody? Why or why not?

If you are referring to songs, I think that there can be no “one melody” It all really depends. If the composer is trying to make one sound more prevalent than others, Yes. There is one melody. BUT, if it is a straight harmony, it all depends on what sound the receiver is hearing. Personally, I am a soprano 1, so in most songs my ears will seek the higher sounds, while an Alto finds the deeper notes.