Speaker not working after backside replacement

Hi there,

I replaced the backside cover of my iphone 3GS and cleaned the inside of the display (dust). I thought everything is working fine until the first call came in. The speaker isn't working anymore! Phoning with headphones or handsfree-speaker is possible, but the little speaker in the display itself is dead. I re-opened the phone and checked all connections - everything is fine. Any hints where I can look at? What may have caused this problem? Which connetors are responsible for the speaker?

Thank you for your help and best regards - TJ