Iphone 3G sound BUG

whem i surf with safari and also at the same time run ipod on the IPhone,
after some time my Ihone stop play sound and display volume (headphones) even no headphones where connected. If i connect Headphones he playes that sound with no Problems over headphones like it is written on the display. It also became little bit hot. I already send my Iphone away for repair... it was 6 weaks away and came back with the same bug.
Ipod Crash while surf with Safari. If i go to Perferences and choose another rington he still play that sound Strange thing.
Could it be caused by a defect in the headphone connector ? The technican which repaired my iphone told me it was a software Problem ...ot
Please need your help cause iam not far away from trowing it out of the window ...
MBP 2.17 Core 2 Duo Mac OS X (10.6.1) Iphone 3g 16g