Anyhow, yesterday I was thinking about the phrase "smit with the love of sacred song," a little later in the Invocation to Book 3:
Yet not the more
Cease I to wander where the Muses haunt
Cleer Spring, or shadie Grove, or Sunnie Hill,
Smit with the love of sacred Song...
A student had read that to mean that Milton (or the Muses: the ambiguity is important because whichever of the the two the participle is modifying, each meaning entails the other: how the Muses feel is how Milton feels)--; I say (I revel in my own disgust for that Victorian, indeed Arnoldian pickup after a parenthesis)--; a student, I say, paraphrased that to mean that Milton had finally come to a place where song could be sacred. He's smit by the love of a certain kind of song, that kind of song we could call "sacred."
How was I to answer that student? The Derridean dodge is right: "song insofar as it is sacred," which just preserves the ambiguity. But later it occurred to me that it's the stresses that make it clear that sacred modifies the concept of song, rather than picking out some songs instead of others. As in Leonard Cohen's "holy game of poker": the game of poker is always holy.
Generally, in iambic pentameter, the five stresses can be ordered in intensity, and there are some loose but real rules as to which of the five can be stressed the most. The last stressed syllable is a candidate for strongest stress (it's often a summation, after all, which is what makes Byron's "Laureate"/"Tory at / Last" rhyme funny: the summative intensities of the stressed syllables in the rhyme don't match except through a readerly force of will or body English). So is the penultimate stress: "and all our woe"; "the ways of God to man." Bolded here means more stressed, if only slightly so, than the other stressed words (woe, ways, man). You may not agree, and maybe shouldn't but it's a plausible reading, because the stress in the fourth foot can be most prominent, as can the stress in the fifth foot, and indeed the stress in the first, especially in a choriamb:
Mee of these
Nor skilld nor studious, higher Argument
Remaines, sufficient of it self to raise
That name, unless an age too late, or cold
Climat, or Years damp my intended wing 
Deprest, and much they may, if all be mine,
Not Hers who brings it nightly to my Ear.
Line 45 is amazing. Probably first syllable gets the most stress, though Years might as well. But unstressed damp seems more stressed to me than even Years does. The least stressed of the syllables in strong position is my, which only receives its official stress by contrast with the in (inintended) that it precedes.
Oh, once you start embarking on the description of metrical effects, there's no end to it, so I won't say anything more about this extraordinary passage. Let's return to "sacred song." The meter makes it virtually impossible, and certainly risible, to read the line so as to give sac more stress than Song. (The capitalizations are almost certainly not officially Milton's, but on the other hand they may well record how he recited the line to his amanuensis-daughters.) The choriambic start to the line gives its center a metrical topography in which the stresses have to go something like this, with 5 as the most stressed syllable, 1 as the least stressed:
3 5 1 4 2
Smit with the love of sacred Song; but chief...
Some interchange is possible, but I feel certain that sacred gets the least stress in the line. After the two unstressed syllables in the middle of the choriamb, love is particularly marked (it doesn't even need capitalization!), and so the instantaneous resources of our sobvocalizing capacity to stress what we're reading are momentarily depleted, depleted for a beat, until we get to Song and the pause, the semicolon, that contributes its own brief, summative finality to the stress. This is the rhythm of meter itself; I suspect it follows something like a pink noise pattern, a kind of miniature version of the rhythm of cinematic cuts, and perhaps of a wide range of phenomena. It seems (I continue to speculate wildly) related to hyperbolic discounting, to a temporal rhythm of attention to and desire for stimulus including that of stress and the cognitive information that stressed syllables tend to offer. It is part of what we call style, part of a great poet's style, to convey meaning through the rhythms of stress itself. That's what makes song sacred.
At any rate, that's why I think my student was wrong, and why even the songs in hell, therefore, are part of the sacred category of song, making hell grant what love did seek.