Pull-down resistor value for interrupts
An Arduino running at 5V needs at least 3.0V on the pin to read it as rising high.  You probably want to go a little more than that.  So that means your lowest voltage has to be like 3.3V or so.  

Note that the analog pins don't have an external interrupt, you'll have to use pin change interrupts and then read with analogRead in the ISR.  So you can't use "RISING".  The pin change interrupts act on any edge.  So they'll trigger when falling as well.  

That's a pretty poor way to handle these buttons.  Normally one would just poll buttons like this.  If your loop takes so long that you might miss a human pressing a button then perhaps you should look at that instead.