How important is impedance matching in audio applications?
Impedance matching is not used in modern audio electronics.

A mic output might be around 600 Ω, while mic preamp inputs are 1 kΩ or more.
A line output will be something like 100 Ω, while a line input is more like 10 kΩ.
A loudspeaker amplifier will be less than 0.5 Ω, while loudspeakers are more like 4 Ω.
A guitar output might be 100 kΩ, while a guitar amp input is at least 1 MΩ.

In all these cases, the load impedance is significantly larger than the source; they are not matched.  This configuration maximizes fidelity.
Impedance matching was used in the telephone systems that audio systems evolved from, and was (sometimes?) used in vacuum tube amplifiers, but even then, it's a trade-off between maximum power and maximum fidelity.
Transmission line effects don't apply.  With a wavelength of at least 10 km (for 20 kHz), I think the most effect you'd ever see from reflection is some comb filtering (HF roll-off) with lines a few km long?  But that's totally unrealistic.
Bill Whitlock:

Audio cables are NOT transmission lines. Marketing hype for exotic cables often invokes
classic transmission line theory and implies that nano-second response is somehow important.
Real physics reminds us that audio cables do not begin to exhibit transmission-line effects in the
engineering sense until they reach about 4,000 feet in physical length.

Maximum power theorem doesn't apply, since:

low-level signals are carried by voltage, not power
high-level amplifier output impedances can be made small compared to the speakers they drive.  Maximum power theorem is for matching loads to sources, not matching sources to loads.

Rane Corporation:

Impedance matching went out with vacuum tubes, Edsels and beehive hairdos. Modern transistor and op-amp stages do not require impedance matching. If done, impedance matching degrades audio performance.

For why impedance matching is not necessary (and, in fact, hurtful) in pro audio applications, see William B. Snow, &quot;Impedance -- Matched or Optimum&quot; [written in 1957!], Sound Reinforcement: An Anthology, edited by David L. Klepper (Audio Engineering Society, NY, 1978, pp. G-9 - G-13), and the RaneNote Unity Gain and Impedance Matching: Strange Bedfellows.

Shure Brothers:

For audio circuits, is it important to match impedance?
Not any more. In the early part of the 20th century, it was important to match impedance. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. Impedance matching reduced the number of vacuum tube amplifiers needed, which were expensive, bulky, and heat producing.
In 1948, Bell Laboratories invented the transistor — a cheap, small, efficient amplifier. The transistor utilizes maximum voltage transfer more efficiently than maximum power transfer. For maximum voltage transfer, the destination device (called the &quot;load&quot;) should have an impedance of at least ten times that of the sending device (called the &quot;source&quot;). This is known as BRIDGING. Bridging is the most common circuit configuration when connecting audio devices. With modern audio circuits, matching impedances can actually degrade audio performance.

It's a common misconception.  HyperPhysics used to show an 8 ohm amplifier output, but they've improved the page since.  Electronics Design showed an 8 ohm amplifier output for a long time, but they've finally fixed it after a bunch of complaints in the comments section:

Therefore, unless you're the telephone company with mile-long cables,
source and load impedances do not need to be matched ... to 600 ohms
or any other impedance. --- Bill Whitlock, president &amp; chief engineer
of Jensen Transformers, Inc. and AES Life Fellow.