I found a thread about the very same problem here : https://discussions.apple.com/thread/3470125?start=30&tstart=0 Last message in thread contains a plausible explanation of the problem : due to corrosion in the USB connector the phone believes that it's docked. Indeed I tried blowing hard in...

Hello, A few days ago a problem appeared with my new iPhone 4S running iOS 5.0.1. The speaker delivers sound in some situations, but not (or erratically) in others. These work properly : ringtone (phone) ; timer and wake-up alarm (clock app). These don't : * SMS alerts - most of the time...