You can roughly approximate the 'loudness' of a sampled waveform by averaging the squares of the differences between samples n and n+1. This will give you a rough indicator of how "loud" these samples will appear to the hearer.
The method is more sensitive to high frequencies than low ones, thats why it can be off quite a bit if the sound has a very extreme frequency distribution.
For a precise solution you will need to take the FFT approach and also correct the extracted frequencies weighting by a model representing the hearers ear (not all frequencies feel equally loud at the same DB level).