What is the metaphor in lines 7-9 of the poem "Ode to the West Wind"?

I can see why you're having problems. Because of Shelley's love of enjambment (continuing the sense of a line onto the next line) the poetic devices actually extend back further than line 7, and on further than line 9.

O thou,
Who chariotest to their dark wintry bed

The winged seeds, where they lie cold and low...

That's the first sense unit. Shelley addresses the wind (O thou) who acts like a "chariot", taking the seeds to their "bed" (of soil, we assume) where they lie cold and low. Then he picks up the pun on bed (human bed / flowerbed) and takes it further:

Each like a corpse within its grave, until
Thine azure sister of the Spring shall blow

Her clarion o'er the dreaming earth, and fill
(Driving sweet buds like flocks to feed in air)
With living hues and odours plain and hill...

Each seed lies like a corpse in the grave, until the wind's blue ("azure") sister, "the Spring", blows her trumpet ("clarion") over the earth. This metaphorical trumpet fills both plain and hill full of living colours ("hues") and smells ("odours").

SO the simile compares the seed lying in a seedbed to a grave, until the trumpet of Spring (this is the metaphor!) fills the "corpse" full of life. It's basically a reference to Revelation in the Bible, when God is supposed to raise up the dead to redemption.