The sounds (keyboard clicks, send mail sound, text & e-mail received sounds) disappear when the iPhone 3G is operating on the 3G network -- it is fine under the wireless network.
Help, please. I missed a couple of important txt's this weekend.
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This sounds like some sort of software corruption. There is no option I know of to disable specific sounds when on//off 3G. Have you tried turning your device off and on?
You may need to perform a restore using iTunes if the issue persists.