Options for In wall center speaker where the stud is blocking cavity

That does present a some placement issues. Here are the few options I can think of, normally I'd say center your TV and center channel on the middle of two studs in your case you still have the Radon pipe to worry about so you cant just move everything to the left by 8"

1) Have you considered moving to an on wall speaker? I know there are a few companies that make pretty clean looking on wall speakers, Axiom Audio, NHT, etc. Typically the don't have great bass response, but if you pare them with a subwoofer they could be satisfactory.

2) Consider using one or two 5" 2-way in walls for the center channel. If you move the whole thing left by 6" it looks like you could fit a single 2-way in between the Radon pipe and the stud to the right. Or you could leave it where is it and use two 5" 2-ways on each side of the stud, or one above on below. You'd need to wire them in series or parallel depending upon their impedance and your receiver.

3) This doesn't look like a very big setup, the front sound stage is pretty compact. You would get great results just going without a center channel and instead set it up as a phantom center. If you're RL speakers are have good midrange you would probably get the best experience out of this anyways.

My recommendation would be option #3. Put the money saved from the center into higher quality RL and run phantom. Phantom centers work great when viewing straight on, as you view from either side the center of sound moves to the nearest speaker, which can be annoying. With bookshelf's and towers you can overcome this by toeing in your speakers significantly (30 deg or so). If you go this route, and you will be viewing off axis, you may want to look for speakers that can be aimed, many can be aimed up to 15 degrees.
Edited by Daniel Smith - 11/30/12 at 1:17pm

Cut a section of the stud out where you want the center speaker. Then cut 2 30 3/4" 2x4's and brace horizontally to the adjacent studs while making contact with the stud you cut. Screw into the cut stud first, then the adjacent studs.

Sounds like it would work to me. I'd mount the crossover board on standoffs to the stud in the wall and put longer wires with in-line spade connectors to each of the speaker components.

The back of that speaker looks nearly identical to some of the monoprice in walls I've used. I'm guessing that one sounds better, but the monoprice ones are a great value.