[Al Skierkiewicz]

You will ned an amplifier to bring up the mic level (~-50dBm ref 1 mW) up to a usable level. A simple precision rectifier would then give you an average DC level which you could then sample to get an idea of the sound level over the long term. A little fudging in software and you could get a good approximation of SPL based on your sample. Remember that most SPL measurements are weighted anyway (frequency limited). I would guess that a logrithmic approximation times 2 would be about right. Of course some serious high pass filtering in the mic preamp will be needed to keep robot crashes, um, contacts out of your measurement.
BTW, to my ears, on the field sound is at least 10-15 db quieter than in the stands. Remember the PA is aimed at the crowd, not the field.