Now I can easily add hit and miss sounds to my button testing app. Will update soon.

Are you making touch changes in M5Sound as well?

To make this satisfying, I needed to be able to move my finger from key to key (glissando), which M5Button didn't let me do. Enter M5.Buttons.pianoMode = true. Now you can slip off one button and onto the next. The price to pay is that gesture recognition is switched off in pianoMode.

(The DTMF dialer is also cooler in pianoMode, so changed that too.)

Also there's now 1000_hzand C_major examples, just so that not all examples are somewhat advanced bits of code. But Piano, like the DTMF_dialer, is still pretty small and quite readable for a stylish polyphonic piano:

/*

Polyphonic synthesizer. Lingering notes will continue to sound as new notes
are played. If you place one finger high on the screen and another low, you
can even play two notes at the same time. As you can see you can have many
synths going at the same time.

*/

#include <M5Core2.h>

#define WKW         40
#define WKH        240
#define BKW         30
#define BKH        160
#define WHITE_KEY  {WHITE, NODRAW, BLACK}
#define BLACK_KEY  {BLACK, NODRAW, NODRAW}
#define NUM_KEYS    14

float notes[NUM_KEYS] = { NOTE_F4 , NOTE_G4 , NOTE_A4 , NOTE_B4 , NOTE_C5,
                          NOTE_D5 , NOTE_E5 , NOTE_F5 , NOTE_Gb4, NOTE_Ab4,
                          NOTE_Bb4, NOTE_Db5, NOTE_Eb5, NOTE_Gb5 };

// (waveform, freq, attack, decay, sustain, release)
Synth synth[NUM_KEYS] = Synth(TRIANGLE, 0, 50, 300, 0.7, 1000);

Button f_(   0, 0, WKW, WKH, false, "f" , WHITE_KEY);
Button g_(  40, 0, WKW, WKH, false, "g" , WHITE_KEY);
Button A_(  80, 0, WKW, WKH, false, "A" , WHITE_KEY);
Button B_( 120, 0, WKW, WKH, false, "B" , WHITE_KEY);
Button C_( 160, 0, WKW, WKH, false, "C" , WHITE_KEY);
Button D_( 200, 0, WKW, WKH, false, "D" , WHITE_KEY);
Button E_( 240, 0, WKW, WKH, false, "E" , WHITE_KEY);
Button F_( 280, 0, WKW, WKH, false, "F" , WHITE_KEY);
Button gb(  25, 0, BKW, BKH, false, "gb", BLACK_KEY);
Button Ab(  65, 0, BKW, BKH, false, "Ab", BLACK_KEY);
Button Bb( 105, 0, BKW, BKH, false, "Bb", BLACK_KEY);
Button Db( 185, 0, BKW, BKH, false, "Db", BLACK_KEY);
Button Eb( 225, 0, BKW, BKH, false, "Eb", BLACK_KEY);
Button Gb( 305, 0,  15, BKH, false, "Gb", BLACK_KEY);

void setup() {
  M5.begin();
  M5.Buttons.draw();

  // Prettier with top of keys as straight line
  M5.Lcd.fillRect(0, 0, 320, 5, BLACK);

  // Trick to make sure buttons do not draw over eachother anymore
  M5.Buttons.drawFn = nullptr;

  // So that you can swipe from one button to another
  M5.Buttons.pianoMode = true;

  // Set up syths with their notes
  for (uint8_t n = 0; n < 14; n++) {
    synth[n].freq = notes[n];
  }

  M5.Buttons.addHandler(pressKey  , E_TOUCH);
  M5.Buttons.addHandler(releaseKey, E_RELEASE);
}

void loop() {
  M5.update();
}

void pressKey(Event& e) {
  // instanceIndex() -4 because of background, BtnA, BtnB and BtnC.
  uint8_t key = e.button->instanceIndex() - 4;
  synth[key].start();
}


void releaseKey(Event& e) {
  uint8_t key = e.button->instanceIndex() - 4;
  synth[key].stop();
}

bool checkRotation(uint16_t msec) {
  if (millis() - rotationLastChecked < msec) return false;
  rotationLastChecked = millis();
  const float threshold = 0.85;
  float ax, ay, az;
  M5.IMU.getAccelData(&ax, &ay, &az);
  uint8_t newRotation;
  if      (ay >  threshold) newRotation = 1;
  else if (ay < -threshold) newRotation = 3;
  else if (ax >  threshold) newRotation = 2;
  else if (ax < -threshold) newRotation = 0;
  else return false;
  if (M5.Lcd.rotation == newRotation) return false;
  columns = newRotation % 2 ? 4 : 3;
  M5.Lcd.clearDisplay();
  M5.Lcd.setRotation(newRotation);
  return true;
}

All it needs is a global uint32_t rotationLastChecked and for you to have ran M5.IMU.Init() (which seems to set power things on the MPU, so there's probably a reason it's not in M5.begin(), haven't read the datasheet.)

checkRotation returns a bool and takes the number of milliseconds between checks. It returns true if it has already rotated and cleared the display for you and needs you to set things up again. So my loop() now does if (checkRotation(1000)) doButtons()

☑ Valid DTMF. (Didn't have an easy means of testing...)

never resting - it seems. Very nice! Just tried it successfully with my old rotary phone.

Cheers
Felix