The maximum value of an integer (which is signed) is 2147483647. If that value overflows, an exception is thrown to prevent unexpected behavior of your program.
If that exception wouldn’t be thrown, you’d have a value of -2145629296 for your Volume, which is most probably not wanted.
Solution: Use an Int64 for your volume. With a max value of 9223372036854775807, you’re probably more on the safe side.