What matters with coupling capacitor distortion is the AC signal drop between terminals. DC across the cap doesn't distort the audio. It may change the capacitance a little, but the resting DC doesn't generate distortion. What's the second harmonic, or the third of DC? Zero Hz, right?
If the cap is used for DC blocking as most coupling caps are, it should be large enough that both terminals go up and down together, and there is no AC signal across the cap. The signal is passed undistorted. Incoming signals below the passband DO create a voltage change across the cap, and suffer distortion, but they are being deliberately attenuated, and attenuation is certainly distortion.
What may be a concern is signals below the passband modulating signals within the passband. This sort of thing happens in low and high pass filters, equalizers, and other tone shaping circuits. In these applications, there is an AC voltage between terminals of the cap, and the potential for varying capacitance. The measured capacitance change is a small percentage between zero volts and full rated voltage. One thing you will notice in every article where someone is measuring capacitor distortion in audio amplifiers - the biggest item the author talks about is his test rig, and how hard it is to get a measuring setup clean enough to detect the distortion.
OK, so let's spitball Jules' MLCC output caps. Given they are 22uF 35V rated. They see 22VDC across them in operation. Fine, the DC might have reduced the capacitance from 22uF to 21.9uF, but it isn't changing. No distortion. The mic output is maybe 100mV with LOUD audio, so this is causing the cap to swing from 22V to 22.1V, or under 0.5%. Let's look at worst case, deep bass. Let's see how 40Hz fares. We get 1/2 power, 3dB down, or 70.7mV out at 9Hz. AC drop across the cap is 29.3mV. According to my calculator that makes the load at the preamp 800 ohms. That seems low to me, but let's stick with Jules' numbers. At 40Hz we're dealing with 2.5 mV AC drop and capacitance change between 22V and 22.0025V on the cap's C/V curve is negligible. The delta in capacitance is from 21.9uF to 21.8999...9 uF, which is gonna change the LF rolloff by ... aahhhh, never mind ... that's way off in the weeds. With a higher preamp input impedance, bass rolloff and distortion will be much less. Jules, did you mean 0.9Hz, I find more typical?
Microphonics can be a problem, but not the way you're thinking. High-K dielectric caps are tiny, inefficient ceramic transducers; think of old crystal mics and earphones from the 1950s, or ceramic transducers generally. Flexing the PCB will generate a voltage, and changing the voltage across the cap will cause it to bend. In DC-DC converter applications running at low frequencies, the converter may whistle audibly. Worse, the solder joints may crack in time. But microphones are by definition small signal devices. It's good practice to check a mic after building for handling noises or resonances etc. by disconnecting the capsule and tapping on the body, the PCB, etc. Usually such effects are completely inaudible with capsule connected, and aren't worth worrying about, but sometimes it helps while debugging a new build. You may find a cold solder joint or cracked PC trace. Or maybe that priceless NOS silver and germanium doohickey you built around is kinda crackly when not masked by room noise.
I'm not claiming that all caps sound the same, or that differences don't exist. I'm just saying that the "horrible distortion" claimed by some audio pundits just can't be measured. Expectations from building and listening to amps, speakers, etc. often carry over where they don't apply. It's possible to pick the wrong cap, too, and if you've been building stuff long enough, you've probably done so.
Yeah, Jules, I've built a couple of mics with MLCC output caps, and they sound fine. I just never told anyone because ... you know, ...
