DIY speaker: braid vs twist vs straight

Cdc, this is a highly system-dependent decision.If it were just simple resistance, or just skin-effect, the decision would be simple. But the capacitance and inductance of the cables interact with the characteristics of your components and speakers. This is why different people swear by very different cables as being the best. They may be the best, but maybe only on their system. If you are doing DIY, it may be of benefit to try some low-inductance configurations and some high-inductance ones. Also, do the same with capacitance. When you hear the difference between these designs, you can determine which way to move toward your final design. It is much easier to do this with DIY, than to buy 25 different cables to test and see which ones you like. From my experience on interconnects, if you don't have RF problems, stay away from shielding. Sound is better without it. With speaker wires, bare wire terminations are generally better than using termination lugs. I use 30ga solid-core copper wire with no shield for my 1 meter interconnects, and 22ga solid-core copper wire for the 3 meter speaker cables. Single conductor run for each. No twists, braids or anything. I keep them well separated along the entire runs. Teflon tube insulators. I use tube gear, and it is quite fine sounding.

CDC, if you don't have some room in the tubing you will never get the wire through it. You will be very frustrated with the trouble you are going to have as it is. Do not "kink" the wire by pushing too hard on it. If you do, the kink will make it even harder to push the wire through from that point on. The best way I found is to stretch out the tubing totally straight, and clamp the end, so there are no bends or curls. Then carefully push the wire through. When you get to the end, then unclamp the tip of the tubing and push the last inch of wire through it, or cut last bit of the tubing off and grab the tip of the wire with a needle nose plier. It's not as easy as you think. Definitely straighten out the tubing and hold it straight!

Cdc, I used the 22ga radio shack magnet wire on my 3 meter speaker runs. You can go without the tubing because of the protective enamel, but the enamel can get rubbed off and there is a small chance of shorting the wires together. For this reason I used tubing on mine. I also separated the runs by 2 1/2" all the way. With single-conductor runs, capacitance is really not an issue. With wires separated beyond the distance where the fields can interact, inductance is not an issue. I used 3/8" tubing around my wires in order to get mostly air-dielectric, except for the few small contact areas where the wire occasionally touches the tubing. In this case, even the dielectric is not an issue. The tubing provides a sound-wave barrier for the wire, so it will get less vibration, than if it was bare. Since the wire hardly touches the tubing, the tranferred vibration is minimal.
You will not heat up the speaker wires or damage anything. The tables you looked at are about 1% loss tables, and they really bear no relevence to this discussion, where the cable length is under 4 meters. The resistance difference at 4 meters is negligable. There is no risk of damage or fire with this type of wire and a 50 watt amp.

Dekay, my big tubing is from Lowe's. I think it is polyethelene. I would have liked to have gotten Teflon, but I wanted to get the wires done, and didn't want to wait for mail order. Also, I figured that if the wire was not having alot of contact with the tubing that it wouldn't matter as much. I used 22ga, because my reading of the skin-effect literature said that 22ga is still under the skin-effect depth of copper. Single conductor runs reduce smear. To separate the runs, I used plastic Tee nipples about every 18" and connected them with a 2" piece of tube. This creates a kind of "ladder" appearance to the speaker cables, with the 2 runs staying about 2 1/2" apart. I guesstimated that 2 1/2" spacing would eliminate/reduce field interaction between the feed and return. If this is increasing the inductance, I sure can't hear it in any loss of high freq's. They look pretty exotic.I really don't know how much all this affected the sound, as I didn't A/B them without the tubing, and then with it. I just did it because I thought it would work out right. I am happy with the result. I will say this much: If these wires are holding any of my signal back, then I will sh!t bricks when I hear it with better cables. It is sounding REAL good right now.

I have never used silver wire in my speaker cables. I did use it once in a preamp I modded, and it was too bright. If you use a SET amp w/ transformer induced problems like high-end rolloff, it can be used to somewhat compensate for that. I really don't like to use cables for tone controls though. Better to resolve the problems at the root.
I don't know of any inherent advantage/disadvantage with square section wire. You'll just have to take your skin-effect depth measurement off the diagonal. I am not the ultimate cable guru. I just made some DIY stuff that had some good design basis behind it, and it worked out. You can too.