Murali, I am sorry to inform you that Windows 7 is not compatible with the Optiplex 620, that's why you have got the sound problem. Dell does not support Windows 7 on that model. You should have checked their website before installing Windows 7 on it.
You will have to re-install the original operating system that Dell pre-installed at the factory, which was probably Windows XP, Windows 2000, or Red Hat Linux.