bad or no sound

Hello,

I have a MacPro 8core with Snow leopard 10.6.8... I have it for more than 3years without problems and since yesterday the sound of the Led cinema sounds very distorted and sometimes no sound at all.

-unplugging USB from Led display only cures NO SOUND,but the sound is still distorted

-same for restarting / shutting down MAC ... Sound is back but very distorted (using itunes,web....)

-Tried to do SMC reset . (unplugging powercord MacPro for 15 sec,then plug power cord back in and wait 5 secs before turning computer on)...still distorted sound and after some minutes no sound

-Also unplugged power from Led Display while computer still on and plug powercord back in led display but still distorted sound..

-Also changing output devices in system preferences doesn't work : internal speakers work and my PCI-424 work but still distorted sound on Led display..

Please Help,thanks everybody for your time and sorry for crappy english

PS : i didn't install new software or update anything!

Tim W