[1]If you are speaking of a particular device, the source impedance is the output impedance of whatever is connected to the input, and the load impedance is the input impedance of whatever is connected to the output.

[2]You still see some equipment that requires proper loading to provide a flat frequency response. Such a device will be marked "terminate with 600 ohms". If it is not used with a 600 ohm load, a 600 ohm resistor should be connected across the output pins.

[3]A transformer is a device that changes a low voltage, high current signal into a high voltage, low current signal or vice versa (the product of current and voltage remains constant). The impedance of the high current side will be low, and the impedance of the low current side will be high.

[4]A gold connector is a good idea on a power amp, or any device that is exposed to high temperature or humidity.

[5]At audio frequencies, the impedance of a wire is almost all resistance.