I am also having the same issue. Really, it is very annoying and dont have the solution as of now.
Ok guys, mine started working all of sudden. This is what happen. I turned off the Dolby surrond sound and the phone powered off after the battery had run out. I charged it in the off state till fully charged and swtiched it on and the Nokia power on sound came. That was all it has been working since. I can't confirm if this is a solution but this is how my sound started working again. It is worth trying.