how to re flatten soundboard?

I bookmatched the soundboard then sprayed some mineral spitirts to see the curl in the wood. it went from completely flat board to a concaved dome down the center.

do I need to reflaten it straight before going into the thickness sander?

Try spraying the other side to make it warp the other way
If you can flatten the warp with finger pressure then it should be ok in the sander. If its really stiff then you'll probably want to flatten it

Take your wife's hand iron and iron that sucker flat, just don't get caught.