If you're building your case out of wood, why not build it like a speaker cabinet (MDF with wood-or-whaever laminate).
Baffle the intake and exhaust - and line the baffles with sound absorbent material - maybe foam like
this stuff from Parts Express. Speaker builders have a lot of the same issues you're up aginst- keeping unwanted sounds inside a box, converting 'em to heat.
Of course having a sometimes-flooding basement to use for a home theater would be an
excellent reason to go with a front projector (ceiling mount) and wall-mounted screen ("have to keep everything above the water line, dear...").
On the other hand, maybe you should be building in fiberglass or marine plywood, because of the high humidity.