hmmm....i have never tried this, but if i must do that one time, the first think is to search a REAL formula to calculate this. Maybee it is enought to search the hightest and lowest amplitudes...but in 99% the lowest will be 0......hmmm....
http://en.wikipedia.org/wiki/Amplitude
but if you must work with "clear" datas (raw, wav, iff) you can use Bass to decode (ogg, mp3,...)
http://en.wikipedia.org/wiki/Amplitude
but if you must work with "clear" datas (raw, wav, iff) you can use Bass to decode (ogg, mp3,...)