I think I've got it! What about 'airy voices'? (Emily Climbs, chapter...er, somewhere near the end; can't go check because a small furry animal is napping on me.)

I've always loved the river Lethe ever since the allusion was done poignantly in Rosemary Sutcliff's The Eagle of the Ninth, and it's always at the back of my mind--I keep wanting to write about it, but I'm never sure I can do it the justice it deserves. I was ecstatic when I found the poem. :D Glad you enjoyed it. (Ack, I sound very clipped and diplomatic. Blame it on sleepiness.)