This is NOT a debate about whether cable A sounds better than cable B, just a (mostly) scientific approach as to what to look for in interconnects and speaker cables.

Any connectors should be tight fitting, with non-corroding or oxidizing contact surfaces, which is why gold is frequently used, though in varying degrees of purity, thickness, and method of application. The conductor in the cable should be attached to the connector through a high-quality solder joint. High quality means performed at the optimum temperature for the correct length of time, and with the correct amount of solder to allow a new alloy to form that mixes molecules of the base metals of the connector and conductor while using as little of the semi-conductive or non-conductive solder material as possible. Silver content solders have a different plastic state and melting temperature then common lead/tin/nickle/flux core solders, and require different application techniques, but can result in a better joint with less inter-element capacitance.

In an environment with a lot of RF noise, a cable should have shielding running the length of the cable and attached to the connector at ONE END ONLY, preferably the pre-amp/reciever end. This is to avoid a ground path that would negate the shielding. Twisted pair for the signal and return/ground are adequate at audio frequencies, though coaxial is prefered at higher (digital or video) frequencies. XLR is preferable to RCA if possible. There is a misconception that high frequencies travel on the outer skin of the cable, and that silver coating of an analog interconnect or speaker cable enables higher frequencies to travel better. Though it is true that higher frequencies do travel towards the outside of the conductor, these are MICROWAVE frequencies FAR ABOVE audio range.

Conductors should be made of a highly conductive metal, such as copper or silver, and should allow flexibility without breaking, and should be of a gauge large enough to pass the low signal amplitudes with as little attenuation as possible. Cables should have a good quality dielectric that will not change significantly in capacitance with age, light, heat, or current. The jacket should also be of a material that will not degrade significantly under the same stresses. All cable lengths should be kept as short a possible for a myriad of reasons, electrical as well as asthetic. Remember though, the farther from the source and the more demanding the load, the heavier the gauge should be.

Connectors should be WELL MARKED for polarity, especially when they are configured as PAIRS. RED and WHITE are preferable to RED and BLACK, as they are easier to distinguish from each other in a low light environment. Avoid running interconnect cables with power cords that may induce an audible hum. If they must cross each other, it is preferable to do so at an angle. Also, be aware of where power is in walls, especially if you are running any audio cables inside the walls as well. Though it may be easy to use the same conduit or path the electrical wiring uses, don't be tempted! Besides, it is against building codes.

So, there are reasons to spend some money on cables, or to build your own, but don't be lured in by the exotic or wild claims for the reasons a particular brand "sounds better" than another. Any that meets the criteria above should perform exceptionally in any system, from $1,000 to $100,000. Quality of materials and construction should be the factors you use in deciding which cables to use in your system, not exaggerated marketing claims, and you shouldn't have to spend thousands of dollars to connect all your components, regardless of what components you have.