@johnandrewjustice - you code had a few typos but I managed to get it to compile eventually.
If all I want is a loudness meter (I don't care about frequencies, just the global sound level), then all I need is the computed magnitudes, right? The vReal vector.
Starting with the vReal vector, what would be a good measure of overall "loudness"? Raise all elements to the power of two, compute the sum, extract square root? Or just a simple sum of all elements?