After following the setup instructions for the “Adafruit MAX98357 I2S Class-D Mono Amp”, I had this HAT working, The sound level, seemed about right for a reputed 0.5W output. After an hour or so, the audio became overlaid with what sounded like digital noise almost masking the original source. Suspecting software issues, I laid the HAT aside until I found your single line installation instruction.
Fresh installation of Raspian Jessie (2017-1-11), the obligatory update & dist-upgrade, then your single-line install: The sound output is very low, the “left, right,…”. of speaker-test -c2 -t wav being barely perceptible more than 1 foot from the speaker; using Mathematica to generate a pure sinewave tone, the resultant is horribly noisy and distorted. Is this more likely to be hardware or software related?