In wall cable

AQ makes several options that look very good.

Power, speaker or interconnect?

I am just getting ready to install two additional dedicated lines in my sound room. I bought 200 feet of this. http://www.soundstringcable.com/gen2/Bulk-Delta-3-12-High-Out-put-30-Amp-Power-Supply-Cord-Power-Cord-Cable.htm.

Elescher,...that looks interesting though the link above did not work until I tweaked it a bit;

http://www.soundstringcable.com/gen2/Bulk-Delta-3-12-High-Out-put-30-Amp-Power-Supply-Cord-Power-Cord-Cable.htm

It looks in-wall rated at seems to be 10-gauge. Let us know how it works for you as it could be a nice, more cost effective alternative to Romex.

Analysis Plus

The link to the soundstring cable looks like 3 conductor, which seems like a waste. There 2 conductor bulk cable is

http://www.soundstringcable.com/gen2/Bulk-Gamma-2-12-standard-speaker-cables.htm

Belden 12 gauge CL3 in-wall is a standard, non-exotic cable. Parts Express and Blue Jeans have it and you should be able to find it locally at an electric supply house. 10 gauge also available.