matching input and output impedance in an audio circuit
We typically don't match impedances in audio circuits.  For example, the output impedance of an audio preamp is typically relatively low whilst the input impedance of an audio power amp is typically relatively high.

The output impedance of an audio power amplifier is typically very low whilst the nominal impedance of a speaker is of the order of 8 ohms.  

While any EE is probably familiar with the maximum power transfer theorem, which shows that maximum power transfer occurs when the load equals (or is conjugate to) the output impedance, it does not follow that the the output impedance ought to match the load.

In the first case, the output impedance is fixed and the load is the variable.  In the second case, the load is fixed and, in that case, maximum power transfer occurs when the output impedance is zero, i.e., it is not desirable to match the impedances.