Capacitor selection - effect of reducing size of the MLCC capacitor in an audio application
There are several 47µF caps on the schematics:


Power supply decoupling (C15 C16)


Capacitance of Class 2 ceramic dielectrics decreases with increasing electric field, and the field is proportional to voltage/thickness. You can use a better dielectric (X7R) that holds its capacitance better for example. You have to determine how much capacitance you actually need, and then choose what caps you will use to get that capacitance at the actual voltage you use. If you want accurate capacitance vs voltage graphs you can try murata simsurfing to plot curves and compare various caps (check "capacitance vs DC bias" in the menu), here is an example:




Headphone output DC blocking cap (C19 C20)


Since MLCC capacitance varies with bias voltage, when used in any kind of filter they will introduce harmonic distortion, sometimes in very "generous" amounts especially at low frequencies. Depending on your requirements, if you want low distortion, you may have to replace the output caps with tantalums. This is why headphone amps chips which do not require output caps are being offered.

Regarding your question, lowering the cap value to use a smaller one would also change the bass cutoff frequency depending on headphone impedance. So you have to account for this. 


Mic Bias filter cap (C14)


MLCC is piezoelectric and microphonic, so using it for mic bias could be trouble... although if the microphone is on the same board, it will pick up the board vibrations anyway so perhaps it wouldn't be a problem. To reduce capacitor value, it would be simpler to use a higher value resistor in the RC filter.