Need a good "under carpet" speaker cable

Try Alpha-Core Goertz in copper or silver. High-quality and flat. Check reviews.

If you are going under the carpet, then I assume you would be installing new carpet?

If the floor joists run parallel to the direction you are intending to put in the speaker cable, why not cut a few access holes into the floor?

You can ask any carpenter how to do this and still keep the floor in good shape. I have seen it done with osb and with plywood floors.

You do not need flat cables then, any would do.

I'll second the recommendation of the Goertz "MI" line of speaker cable. They offer 3 models which vary in width. Goertz is one of the real values in high-end speaker cable and interconnects. Their products are grounded in actual science, not "wire mumbo-jumbo". I suggest you read the excellent material on their Web site: http://www.alphacore.com/

I'd recommend the Goertz as well, good stuff but it may not fit your budget. If you'd like something inexpensive QED makes some decent flat cables at roughly half the price of Goertz. The QED stuff strikes me as more rugged as well. In your set up (long runs add up, and it is only for rear HT speakers) I'd save some money and go with the QED. Jeff