No sound in my M1330 after upgrading to Windows 7

I have an XPS M1330 which i recently upgraded to Windows 7 and it works great. Except i have no sound. The speaker icon indicates speakers are working. Sound in Control Panel indicates speakers are enabled. When i press Test, the green bar moves, but there is no sound coming out of the speakers. When i plug in a headphone, i do hear sound. Any idea of what could be causing this issue?

Thanks

OA4