Here is an explanation of the words in this sonnet
concave womb - hollowness, depression in the hillsisde.
re-worded - repeated as an echo.
plaintful - sorrowful.
sistering - adjacent, close by. (A neologism derived from 'sister'. The word suggests not only that the vale was close by, but that the echoing hill was closely related to it, and sympathetic to the story being related.)
spirits - attentiveness, interest.
double voice - the tale itself plus its echo; deception is also implied.
accorded - consented.
list - hearken to, listen to.
sad tuned - sung (related) in tones of melancholy.
Ere long espied - before long I espied, caught sight of.
fickle - unstable, wild, unreliable. This anticipates the behaviour of the maid as she subsequently relates it, but the adjective is more fittingly applied to the man who seduces her.
papers - probably love letters.
full pale - very pale.
a-twain - in two.
Storming her world - making her own life a misery.
sorrow's wind and rain - the sighs and tears of sorrow.