Impedance matching for wired headphones extension?
At audio frequencies, transmission line effects will not be noticed. Maybe if the cable was several km long then yes but, this would be a ridiculous length for a headphone cable.

It is the wavelength that is important and at 20,000 Hz the wavelength is 15 km. A general rule of thumb (in avoiding reflections) is that cables can be unmatched providing the length does not exceed one-tenth of a wave length - that's 1.5 km however, the signal amplitude would be so small at this length that it wouldn't make any difference.

Lower frequencies have a longer (less "problematic") wavelength.