My younger son's involvement with creating popular music and my recent attendance of opera performances has gotten me thinking about how music should support and enhance the lyrics. For an opera, a consistent relationship between music and lyrics and the overall theme of the opera leads to "unity of effect", which is crucial to moving the audience to a desired emotional state. (I am arguing from a traditional perspective. If the goal is to bewilder the audience, then a persistent "disunity of effect" is probably the weapon of choice.) Even with popular songs, a careful crafting of music can lift a workmanlike lyric into the realm of art.
Take, for example, the Paul McCartney song Yesterday. Even from the beginning, the descending notes of the dactyl "Yesterday" sound like the sigh of a lament. These dactylic sighs close off each verse: "game to play", "hide away", etc. The rising notes during the chorus for "Why she had to go" and "I did something wrong" reinforce the lyric's pangs of regret. Even without the self-consciously "artsy" string quartet orchestration, this would still have been one of McCartney's most artistically satisfying achievements. (One of his least satisfying is the dreadful Ebony and Ivory, where the music and lyrics fight each other.) I'm looking forward to the time where my son can use his language ability and his composition skills to construct a song where the music and lyrics are so tightly coupled that every note and syllable seem inevitable.
Edgar Allan Poe thought a lot about "unity of effect." I'll let him have the last word:
"I prefer commencing with the consideration of an effect. Keeping originality always in view — for he is false to himself who ventures to dispense with so obvious and so easily attainable a source of interest — I say to myself, in the first place, "Of the innumerable effects, or impressions, of which the heart, the intellect, or (more generally) the soul is susceptible, what one shall I, on the present occasion, select?" Having chosen a novel, first, and secondly a vivid effect, I consider whether it can best be wrought by incident or tone — whether by ordinary incidents and peculiar tone, or the converse, or by peculiarity both of incident and tone — afterward looking about me (or rather within) for such combinations of event, or tone, as shall best aid me in the construction of the effect."
