No sound while playing music

Hi,

My IPhone is doing strange...

When I play music I do not hear anything music via the internal speaker while I see the song playing in the screen

The ➕ and ➖ knobs on the left, for increasing the volume of the music, are also not responding!!!

My phone is working perfect including the internal speaker, and the the ➕ and ➖ knobs for volume works also fine

When I plugin my headphone, I do hear music and then I can use also the ➕ and ➖ knobs!!??!?!?

Anyone regonize this beaviour?

gr,

Paul