Radio Frequencies: When an RF voltage and current are transmitted along a wire, the impedance of the cable itself becomes significant, and for any distance that is "significant" - which is to say any distance greater than about 0.1 of the signal's wavelength - matching is necessary. The wavelength is calculated from the speed of light (3 x 10 ^ 8 m/s, or 300,000km/s) multiplied by the "velocity factor" of the cable. This varies from about 0.7 up to 0.9 depending on the dielectric constant of the inner insulator and cable construction, meaning that a signal travels more slowly in a cable than in free air or space.
Wavelength = C / f (where C = velocity and f = frequency)
A 1Mhz signal travelling in a typical coaxial cable (velocity factor of 0.8) will have a wavelength of ...
Wavelength = ( ( 3 x 10E8) x 0.8 ) / 1 x 10E6 = 240m
Based on this, any attempt to transport a 1MHz signal further than about 24m will start to cause problems unless the send and receive impedances are properly matched - not only to each other, but to the cable as well.
In the hi-fi audio world, this is not an issue, since this is 50 times the highest frequency we can hear, and few instruments create appreciable harmonics above 20kHz anyway. In theory, we could send an audio signal 12km without having to worry about impedance matching, although at extreme line lengths matching can reduce high frequency signal losses. To understand the reasons is beyond this article, as it involves transmission line theory - not one of the easiest concepts to grasp.