You are correct, the 3rd headphone jack has smt caps and resistors (along with it being a switching jack) so that it can be a filtered ProSound output. And the final design will have ground planes on each of the boards for increased interference reduction.
FYI those aren't resistors on the filter board, they're inductors.
Oh wow, looking back at the schematic, you're absolutely right. Luckily it shouldn't be hard to find inductors with a 0805 footprint. Thanks for the tip!