Are you using powered external speakers? Have you checked to see if it is a speaker issue?
I have a Sony Vaio RS520 Desktop, P4 with 1.5GB memory, SoundMax sound card; OS: Windows XP_sp2. About a year ago I noticed that my sound would stop after running Photoshop. Recently, however, whenever I get the sound to play--usually after the initial bootup--if I turn off the sound playing program then try to play something again later it doesn't work. (Now it doesn't matter if I have Photoshop or any other program running.) I have checked the Device Manager when there is no sound, but it indicates no problems. I have gone to Sony's website and there is not a driver update. I am at a loss as to what the problem is or how to fix it.