Yes sound will go right through the foam. It was meant as a solution to condensation. If you've already got them insulated and you think that insulation is also water tight, then that's good enough.
Sound will also go right through the pipes. It'll go through the pipes less if they are mechanically decoupled in some way. But the best choice is to keep the sound away from the pipes by building a little bitty wall around them.
Outside a 'soffit' is the under board of your roof, as shown in
But inside, which is what I meant, was a 'soffit' that is just a long box that that is near the ceiling and runs the length of the wall. Most often it encloses ductwork.
Page 15 of this shows duct vents in a soffit that has the main ducts in it.
This shows how to make a very thin soffit, in this case just to hold some wire.
http://www.diynet.com/diy/hi_family_roo ... 64,00.html
Here's how to build a box beam, which if you run things inside of it, is sort of like a little soffit.
http://www.diynet.com/diy/hi_kitchens/a ... 01,00.html
'Soffit' can also be something unusual that just looks like a soffit.