This is getting to be very difficult, mainly because it is swamped in surmise. Poeple are surmising left right and centre about historical practices involving poetry and song, about which we in fact know very little beyond the increasingly obvious fact that poetry used to involve a lot more singing than it does now. (We often enough know that things which survive as poems were or could be sung, but we don't know what the music was like, which would make a lot of difference. This is the case with Blake for instance.It is a fair guess that they were folk song tunes or old hymn tunes (often the same thing) but who knows?)

And for me a lot of it goes against the grain because it is dedicated (as Jamie said) to problematising and diversifying what I increasingly insist on trying to see as a unified thing. I go back to my insistence that poetry and song (lyric) employ basically the same skills to the same effect and to the same purpose. 

What purpose?  You would not think we were speaking of an art with a purpose, and that the purpose might be to comfort and hearten people against despair, whether by diversion or by unfolding the reality and armoring the spirit with some hope. 

How about this as an exercise to gain clarity?--

How about trying to define what it is that all those poets mean when they title or subtitle a poem with the word "song". What do all these "songs"  which are not sung, but many of which could be by various means, have in common? In English alone there must be many hundreds of them. Blake's songs. All those 16th -17th- 18th Century songs obviously set in lyric metre to make them available to music, including hymns -- Donne's Songs and Sonnets -- what's the distinction?  (but beware: "Sonnet" doesn't mean little song, it derives from a diminutive of the Italian for "sound") (but nevertheless clearly remains a lyric concept and Italian sonnets were sung).  And it doesn't have to be explicit: Herrick didn't title his poems like that but the first words of his Preface are "I sing of..."   Neither did Wordsworth but "lyrical" and "ballads" are both song terms. Whitman did, in plenty.   And it continues -- Dream Songs -- Mercian Hymns -- "Singing School" - Cantos -- "A Book of Music with words" --   I do it myself, why? And sometimes the singing is perfectly obvious without a word being said in title or anywhere (Dylan Thomas)  It seems to me to amount to an almost universal recognition that to venture to create a "poem" is to raise the voice into singing mode, with all the attached formalities or effects.   The only division I can clearly see is when "song" is a larger concept derived from epic (Cantos whether by Pound or Byron or Dante)  but they share a lot of the technicalities.  With intermediary modes which create "high" song  (ode, elegy etc)  

Of course there is poetry based on speech, but a careful, dignified, "measured" speech  --Milton, Chaucer (?), Stevens, even later Graham. But, being poetry, "measured",  which is where the ghosts of music, and dance, enter.  But there is also, a lot of it nowadays, poetry which deliberately, through alienation or anger or theoretical cussedness or whatever, denies the lyrical entirely and writes the anti-song, which must be the anti-poem, substituting ugliness for attraction.  Like, like who?  I'm not sure.  Olson perhaps. Larkin. (slight giggle here)

Sorry I can't respond to a lot of those other responses, time crushes things out.  But most problems resolve themselves in the end, as most poems clarify themselves eventually.

(On Wyatt see Denis Stevens, Music and Poetry in the Early Tudor Court (1961), index, which I'm sure you'll already know. Not that contemporary practice need dictate what you do with such a poem. But to be sure of the authenticity of those freedoms which speaking permits, those personalisations, you'd surely also need to know about the condition, the sound, of the spoken language, at that time, especially in recitation, where I think it probably sounded half-way towards music, anyway formal, and perhaps not at all intimate. But it is difficult to get this kind of information. Or you can be perfectly happy to make it "your poem" --  no problem with that.)

PR