Song lyrics and the "meaning" of music
The problem that the songwriter has, when writing song lyrics, is to find words where the meaning of the words is consistent with the "feel" of the music.
The choices that songwriters make when writing lyrics might be able to tell us something about what music itself "means".
Or perhaps these choices might tell us something about what music used to mean, on the assumption that song lyrics are filling in the meaning that used to be contained in the music itself, at a time when music had a fuller meaning and words did not exist at all.
Also, if we can develop a suitable theory of the "meaning" of music, this might tell us something about what is the best way to write song lyrics.
The multiple purposes of song lyrics
One problem with studying song lyrics, in order to understand the nature of music, is that some song lyrics have purposes of their own which are not necessarily subordinate to the musicality of the music.
For example, any sort of humour in lyrics may be telling us more about what humour is that it is telling us something about the music.
If there is some subset of song lyrics which are not fully subordinate to the musicality of the music, there may be no a priori criterion that reliably distinguishes these song lyrics from those that are fully subordinate to the music itself.
The most we can do is to classify distinct categories of lyrics, and hope that some particular category of lyrics corresponds to a plausible theory of what the meaning of music may have been in the past, assuming that music came into existence before the appearance of word-based language.