The only standard I know is that all long audio leads are designed as a balanced 600 ohm impedance. With ultra long audio leads, such as overhead telephone leads, they are so touchy about any change of impedance that compensation is used for the core identification marks. So that pair one, might have a black marker every 100mm, pair two, two marks every 200mm, pair three, three marks every 300mm. So that in a kilometer every pair has the same number of markers. For long music leads, the balanced pair is enclosed in a screen and all the inputs are terminated in 600 ohms, though the outputs are normally low impedance.
Frank