House wiring for dedicated power to stereo

As you probably know, THHN is a good general purpose building wire - stranded bare copper with PVC insulation and Nylon jacket designed for applications utilizing less than 600 volts. Ten-gauge is pretty hefty, but it will certainly work fine as a dedicated line for your system. Make darn sure you run it all the way back to the fuse box though, as the rest of your house is likely wired with 14 or 12. A dedicated 12G line would suffice just fine, and 12G is easier to work with than 10G. I used 10G for a dedicated line in a previous home and today I utilize a dedicated line with 12G - darned if I can tell any differences.

12awg cable is the min for a 20amp 120VAC outlet.
10awg cable is the min for a 30amp outlet but you can use it on a 20amp rated outlet-just ensure that the breaker is also only 20amps.

Higher guage may or may not provide any audible benefits, but will play a factor depending on how long the ruin is between the outlet and the Main panel. The longer the run, the more resistance, and thus the more voltage drop on the line. Of course the VDrop is dependent on the amount of current being pulled at the moment.

If you have a short run, 12 will suffice just fine. However if your run is like 100', while 12awg may suffice 10awg may be better.
In my case, my outlet was about 100' away and I had a huge VDrop-of like 11 volts when pulling about 11 amps.

Since you're going through the effort/expense, and since you ordered 2 outlets, if they are being installed in the same location, get 10/3 or 12/3 wire. It has a shared neutral and ground and separate hot wires for 2 circuits. If they are being installed in the same gang box, be sure to use a double breaker (usually used for 220V applications). This will ensure that all power is off to the entire box in the event that you or somebody else ever want to work on that outlet in the future.

Oh yeah, by the way, I'm NOT a licensed electrician, but I did stay in a Holiday Inn Express last night ;)

I'd go with two separate runs of 10 gauge. You might not be able to hear a difference between 10 v 12 gauge however you never said how long the run was.