1) Music is a privileged metaphor for poetry itself. This metaphorical function exists in weak and strong forms. The weak form is mere conventional usage, as in references to singing and song that aren't really meant to suggest anything deeper. The strong form sees more profound connections based on the inner kinship between poetry and music. These connections take varying forms depending on the period, so the Renaissance has the music of the spheres, and the symbolists their synesthesia.
2) Aside from this metaphorical usage, we find musicality in prosody. All the sound of poetry and its rhythmic structures. This kind of musicality justifies the strong version of the music metaphor.
3) Finally, musicality refers to the actual connection between music in poetry in their common origins: song itself. Vocal music is the primordial form of music, and sung poetry is the primordial form of poetry. In this conception, song is not a hybrid art at all.
Which of these aspects is most interesting? I think the merely conventional metaphorical sense is easy to dispense with, if seen as a kind of dead metaphor. Is song in Whitman's "Song of myself" a dead metaphor? We would have to see. I think the strong version of the metaphor is interesting, along with the historical argument from the primacy of song itself. Prosody is interesting too, for some people like me, but it's more interesting to me, right no, to the extent that it connect with musicality (1) and (3).
[The fourth sense would be what happens when poetry, already existing as such, is set to music, giving rise to a hybrid form... Now we are seeing poetry and music as separate things and seeing what happens when we put them together....].