speaker icon greyed out at boot

Hi, I am new to Macbook Pro 13' (late 2011, I think) and its operating system. I am having problem with the speakers. I had no trouble with it when I bought in December 2011, even when I updated the OS 10.7.4. But when I upgraded its RAM from 4gig to 8, that was when trouble started. Everytime I boot, the speaker icon is visible at the start, but when the wifi and battery icons appear, the speaker icon greys out. No sound come out from the speakers. When I checked the speaker section in the System Preference, the buttons are also greyed out. I would appreciate it very much if someone offers a solution to this glitch (I suppose). Thanks.