I Phone 4 loss of sound.

Hi. My 16GB I Phone 4 from around Jan 2011, running on IOS 6 has suddenly decided that it will not play music or Audiobooks through its own speaker. It plays fine through ear-phones or through a docking station and the volume controls (buttons & slider) work fine. However, as soon as the earphone jack is removed, the volume slider control disappears from the screen immediately and even though the music is still ‘playing’ there is complete silence. The Vol buttons have no effect. Otherwise, the phone works perfectly and plays Voice Memos quite happily through its speaker, its only music & Audiobooks that have a problem.
I’m afraid that any advice kindly offered will have to be in layman’s terms. Updating to IOS 6 pretty much stretched my IT skills to the limit. Any help would be gratefully received.
Cheers, Ordy.