I have the same issue. I have a HP DV9925nr with Vista Home Premium 64bit. A few days ago I had Windows conduct updates and I lost the sound. I thought nothing of it and had Vista update itself to SP2. I had this issue before and after SP2. The sound icon has the "x" over it and lists "no audio output device installed." I have downloaded the Conexant driver several times for my computer and to no avail. I have searched for the speaker device on my computer but it does not exist. When I go to device manager, there is no folder for speakers nor audio. My computer believes that the speakers themselves do not exist. The several remedies that people have posted have offered me nothing. I am not the true computer wiz, but I know that something is seriously wrong with my computer.
*I have also tried using headphones, but I get absolutely nothing out of them.