From what I recall to remove the annoying speedcam sound in iGO8 is as follows: go to the data folder [not data.zip], audio folder in which you will find the different .wav files, find !alert5.wav and rename it to !alert5old.wav. I think this is the way. But I could be wrong.
If you want to change the sound in iGO8, just rename the new sound in the folder to !alert5.wav and remember to rename the other to old so you don't conflict.