Wiring – How to run wires through a wall with existing wall wiring? (Tight Spaces)

Since this is the accepted answer, I can't delete it. But after another decade of DIY work, I agree with the comments below. This is not code, and not safe. Device power cables are not intended to be run in walls and are a fire hazard. IMO, @gregmac's answer below is correct, provided you use proper electrical cables in the wall (as gregmac notes in the comments).

@Jeremy has a good solution, but if you're willing to do a little more work, there is a little better-looking answer. Since you're running an HDMI cable, I assume you have some kind of small stand/table in front of the outlet that your cable box sits in or on.

With a drywall knife, punch out a hole behind the TV and install a plastic "old-work" 1-gang box. Don't worry, we're not going to install electrics here; just a plastic box that has a couple of "ears" on it so it can slide into existing drywall. Do the same thing at the bottom of the wall, above your electrical outlet. You can now run the cables easily through the wall without patching any drywall. If everything's laid out correctly, both will be hidden (one by the TV, one by the stand/table that holds your cable box).

You've got a lot of different questions, so I'll try to answer the ones I can:

1. Coax sucks because of the necessary bend. You're better off going with a backless box like the one you linked to, because it makes running cables so much easier. (if you have a second person, have them push the cables up from below (or a straightened coat hanger or other wire to act as a needle, if the bundle of cables is too floppy), and then snag it from the hole with a wire bent into a hook) ... which you can't do as easily if you mount a box with little punchouts. Also, you then don't have to worry about crowding the box, etc.

2. It's easier to run lots of cables at once. What what you're planning already, I'd likely run an extra cat5e/cat6 cable, and then get an HDMI-over-twisted-pair (aka HDMI-over-cat5) extender if you wanted to go that route, or re-terminate it for RJ11 if you wanted to change it over to phone.

3. I don't know of any other way to get cables/pipes through the sill other than drilling, and it's allowed for plumbing, electrical, etc, so long as you then spray foam the holes afterwards (at least, it's required in my area, might not be in yours).

4. You only need to line up well enough to get it in the same bay, so you have a ~14.5" target. I cut the hole for the box first, then check to verify the location of the studs (I don't use hardware that requires me to attach to a stud, so I just have to make sure to miss it when cutting my hole), then measure off of some reference point that I'll be able to find down below. (electrical runs, walls, etc.) If nothing else, you can use plumbing or electrical for identifying where the walls are from below.

5. To avoid humidity issues, use spray foam to seal around the cables. (see #3). It may even be required by code in your area.