Stabbing in the dark, but here are some thoughts:
* Do any other sounds which normally work, work when the audio notifications don't work? If some do and some don't in those situations, which ones don't?
* Perhaps there is some ill-behaved process running in the background which sometimes interferes with sound. Whenever the problem occurs, go into Task Manager and kill one non-essential process at a time, then test sound again, to see if the problem at one point goes away. If it does, you have likely found the culprit.
* Reboot into Safe Mode with Networking, to see if the problem occurs there. Only the bare essentials run in Safe Mode, so if the problem doesn't exist in Safe Mode with Networking, then it's probably a Windows issue which can be tracked down and disabled, thereby enabling the sound to work.
* This one will take more time and patience than the above: Run MSCONFIG, and hide all Microsoft processes. Disable one or two non-Microsoft, non-essential-looking processes at a time from running at startup; then reboot, and see if the problem has disappeared. If you're able to finally track down the offender in this way, then you simply leave that one item disabled from the startup process.
Good luck.
