i know its not S3 specific at all, but as tall paul says, cold air induction kits really seem to amplify the sound!
my car is currently TOTALLY standard other than a cold air induction kit, and running the standard 7psi, the recirc sound when i back off is really quite loud, the turbo spool up noise is also far more audible with the induction kit in place!
a friend of mine has a mk4 golf gti turbo, running an mbc (perish the thought!) with the boost set to 15psi, and the recirc sound when he backs off is really very loud, not like the low toned flutter of mine, but really a really high pitched release.
personally, i MUCH prefer the sound you get from the diverter valve than any dump valves. every other chavvy turbo car out there runs an atmospheric dump valve, and everyone knows what it is.
when i pull up in mine, and the diverter valve gives off its little flutter, people seem really shocked and interested to know what it is. and its a far nicer sound to hear than a big TSCHHHHHHHHHHHHHHHH!
having my car Revo'd and a forge 007 fitted next month
cant wait to hear it then!