I've been wrestling with this one again too, so... let me get this right, just trying to organise my own thoughts more than anything:
1. birth pains of astral projection was the original 'decoder' song with it's cluuuuuuuuuuhhhhhhhhhhhs but i guess nobody got it so they made the secret song to spell it out some more.
2. byron used the number charts of the four planets to create the sigils for each of the interludes. these sigils in turn were somehow supposed to point to the four songs you were supposed to use to look up the lines in the lyrics (i still don't quite how this part works and feel this may well be the piece of the puzzle that's missing.)
3. the secret song reveals the greek names for these songs, which gives a seemingly easier route to They Aren't All Beautiful, Geography, Stones of October's Sobbing and Sleep Is A Curse via translations of the greek(?)
4. if these are indeed the correct songs, then many of the numbers seem to work and you can get stuff like 'See creation breathe in you' which kinda makes sense and then others which don't seem to work at all and wild numbers like the 400 and 'the the' repeats. so either a) we're reading the numbers wrong as toby hinted b) the numbers are screwed up (Byron did hint at potentially having got it wrong) or c) there is some extra modifier that changes the way you apply the numbers to the lines of the lyrics that we're missing.
5. just an aside but i love how the line numbering is screwed up on They Aren't All Beautiful just to make it even harder!
So...
With a bit of creative arithmetic and slight reordering I came up with:
See creation breathe in you
I speak voiceless sleep
The thoughts clear shivering tombs
So great the humbled life