int not declared in this scope
Declare the updateMux() function by copying it from your first sketch into your second sketch:

void updateMux1 () {
  for (int i = 0; i &lt; 8; i++){
    digitalWrite(pin_Out_S0, HIGH &amp;&amp; (i &amp; B00000001));
    digitalWrite(pin_Out_S1, HIGH &amp;&amp; (i &amp; B00000010));
    digitalWrite(pin_Out_S2, HIGH &amp;&amp; (i &amp; B00000100));
    Mux1_State[i] = analogRead(pin_In_Mux1);
  }
}


And do the conversions on the stored Mux1_State[] values during printing like:

  for(int i = 0; i &lt; 8; i ++) {
    Serial.println(Mux1_State[i]*5.0/1024.0);
    if(i == 7) { 
       Serial.println();
    } else {     
       Serial.print(",");
    }
  }