Use a simple duel op-amp to give some gain to the mic and then drive a the
second op-amp as a presicion rectifier to charge a cap on an analouge input
this is a simple VU meter / PPM meter giving an analouge output to a '71 or
similar. Gives a proportinal voltage to sound presure level (loudness). cost
is low <$0.30

Cheers Steve.....