If i recall correctly windows reloads these drivers every time the computer is booted up. So if there is some sort of conflict, either malware as already mentioned, or something else, the drivers wont load.
I suggest you check of device mgr or anywhere else you can get info on what irq's each device is using and see if there is a conflict. I imagine there is. Audio always wants certain irq's so its best, when building a new computer, to install audio first before video and other drivers so there wont be problems down the road.
In the end, if you can fix it this way, and there are no malware problems, then you may have to uninstall your video and other drivers, install sound, then re-install the others, that may fix the problem once and for all.
Good advice + great people = tek-tips