When the length of an electrical cable is shorter than about 10% of the wavelength of the signal it conveys, the voltage and current are effectively the same at all points along the cable. However, if the cable is longer than 10% of the wavelength, the signal can be considered to propogate as electromagnetic waves along the cable, and this condition is referred to as a 'transmission-line'.
At 20Hz the electrical wavelength is well in excess of 2,000 miles, and even at 20kHz it is over six miles, so there is no need to consider transmission line theory in normal audio interconnections in a studio. However, it is very important in radio-frequency installations as the relevant cable length is about 20cm for 100MHz signals, and just 20mm at 1GHz. In the studio, it is also important for other high frequency signals, such as digital audio and video.
A transmission-line can be constructed in many different physical forms, such as spaced parallel wires or coaxial cables, but all are generally of uniform cross-sectional area and have a defined 'characteristic impedance' per unit length. To prevent the signal being reflected from the end of the cable it must be terminated at both ends with source and destination impedances which match the cable's characteristic impedance.
The transmission line term is often also used (usually erroneously) to describe a form of loudspeaker cabinet design in which the lowest frequencies are guided down an open-ended tube of considerable length, lined with materials which allow the lowest frequencies to pass but absorb the higher frequencies.