Impedance mismatch, as that for coaxial cables and terminations at radio frequency or fast pulses, is not relevant normally to audio, where the signal wavelengths are much longer than the cables.
Power amplifiers (either vacuum-tube with an output transformer or solid-state with a direct connection) are normally designed to supply a constant voltage to the load, over a reasonable range of load impedance. Solid-state amplifiers usually have an extremely low source impedance, much lower than the nominal load impedance. The source impedance from a typical tube amplifier is higher, but still lower than the load impedance.
Lower impedance loads will draw more current than higher impedance loads. The actual load impedance seen by vacuum tubes is somewhat critical to low distortion operation: therefore, the output transformer usually has taps for different popular speaker impedances (4, 8, 16 ohms). With solid-state, the most important question is the power dissipated by the output transistors at higher current than the original design. A solid-state amplifier designed for a 4 ohm load will be safe with an 8 ohm load, but will deliver less power than into a 4 ohm load.