If you're using the new I8910XXIG2 firmware and experiencing a loud hiss or wihistle when playing back recorded video, here's how you can adjust the mic gain.
Go to the secret service menu: *#0002*28346# - then press "3 Records" - then "4 CamcordAMR_AMR� and modify the first value from 30dB to 24dB. Then select Back > Back >Back and finally click on "6 Back-Up" and you will see that Backup appears ok� Now press Exit and the problem will be fixed.