Throughout the Odyssey, Odysseus' journey has been a serious struggle. He has watched men die recklessly, and travelled through treacherous storms. He suffers the wrath of gods, and we readers focus on the poor unfortunate situation of this man and his men.
The form of the sirens is in short stanza poetry whereas the rest of the poem almost reads like prose. These hauntingly flirtatious words celebrate and praise Odysseus the warrior like no other character has done thus far in his journey. This glorifying tone contrasts to much of the gloom Odysseus has encountered so far.
The siren's song is considered a lethal text, in that the mind of he who hears it will be destroyed. (Actually, the mind will not be destroyed, but it will be forever changed, often for the better.) In this case, Odysseus must hear their song so that he suffers, and so that his will to return home will be strengthened by bitter longing. Their song is like the Aristotelian nature of tragedy; it causes a katharsis (purgation of pity and fear) in its listeners. To pay for his past sins, Odysseus must fall in order to rise.
Other great lethal texts include the monoliths in Arthur C. Clarke's and Stanley Kubrick's 2001: A Space Odyssey. They draw in the unsuspected and then generate a shrill, piercing sound that transforms and transports their listeners. Such is the power of art; indeed, great art is a siren's song.
Their song is also metapoetry, or metalyrics. It's poetry about poetry. It's self-referential and self-reflexive. It differs from the rest of the Odyssey, which is narrative (epic) poetry.
Here's the song:
Sweet coupled airs we sing.
No lonely seafarer
Holds clear of entering Our green mirror. (Book 12, 173-176)
Here's the passage about the song:
Square in your ship's path are Sirens, crying
beauty to bewitch men coasting by;
woe to the innocent who hears that sound!
He will not see his lady nor his children
in joy, crowding about him, home from sea;
the Sirens will sing his mind away
on their sweet meadow lolling. There are bones
of dead men rotting in a pile beside them
and flayed skins shrivel around the spot.
keep well to seaward; plug your oarsmen's ears
with beeswax kneaded soft; none of the rest
should hear that song.
But if you wish to listen,
let the men tie you in the lugger, hand
and foot, back to the mast, lashed to the mast,
so you may hear those harpies' thrilling voices;
shout as you will, begging to be untied,
your crew must only twist more line around you
and keep their stroke up, till the singers fade. (Book 12, 41-58)