Help with LCD's menu and seeing values continuously
As Gerben implied in a comment, menuChangeEvent is called only when the menu selection changes.  Thus, your voltmeter readout will print only whenever you change the menu selection to Item2SubItem3.  The code shown below illustrates how to modify your loop() so that it checks the menu selection once per second.  Each second the main loop will do whatever function is currently selected, such as displaying a current voltmeter reading.  Note, you can make the menu decoding shorter and less tedious as shown in the answer, and can put variable declarations near where they are used instead of in random places.

void setup() {
}  // setup()...

// DC voltmeter reading
void volt () {
  const float vPow = 4.7, r1 = 100000, r2 = 10000;
  lcd.clear();
  float v = (analogRead(0) * vPow) / 1024.0;
  float v2 = v / (r2 / (r1 + r2));
  lcd.setCursor (0,0);
  lcd.print("DC Voltmeter");
  lcd.setCursor (0,1);
  lcd.print (v2);
  // Don't delay(1000) since loop() takes care of delay.
}

long int second = 0;
uint8_t itemcode = 0; // Will be number nm for Item n, Subitem m
int passnumber = 0;
void menuChanged(MenuChangeEvent changed) {
  MenuItem menuitem = changed.to; //get the destination menu
  String itemname = menuitem.getName();
  lcd.clear();        // Make printing spaces at line ends unnecessary
  lcd.setCursor(0,1);         //set lcd printing position = second row
  if(newMenuItem.getName() == menu.getRoot()){
    lcd.print("Main Menu");
    itemcode = 0;
  } else {
    lcd.print(itemname);
    itemcode = 10*(itemname[4] &amp; 7);
    if (itemname.length() &gt; 5)
      itemcode += itemname[itemname.length()-1] &amp; 7;
  }
  passnumber = 0; // Actions can test passnumber if they like
  second = 0; // Cause actions to take place now
}

void loop() {
  long int now = millis()/1000; // Get elapsed-seconds
  if (now == second) return;    // Only do stuff once per second
  // Update to current second and do chosen menu item
  second = now;
  switch (itemcode) {
  case  0: break;
  case 10: break;
  case 11: break; // Fill in with code for different cases
  case 12: break;
  case 20: break;
  case 21: break;
  case 22: break;
  case 23:    // Item 2, subitem 3, read voltmeter
    volt();
    break;
  case 30: break;
  case 40: break;
  }
  // You can test passnumber to control for cases that only go once
  ++passnumber;
} // end of loop()


Edit: I moved the declaration of second and added second = 0 within menuChanged() to decrease latency between a menu item being selected and its action taking place.  Without second being zeroed in menuChanged(), up to a second could pass between an item's selection and its action; latency would average 0.5 seconds, which is undesirable in a user interface.