It could be a heat issue causing a component to work intermittently. Often times in electronics something will work when it's cool but stop working when it warms up or will not work when it's cool but then start working when it warms up. These can be some of the hardest problems to diagnose. It's not necessarily overheating, just normal temps causing things to expand and contract. It could be caused by a poor solder joint.
If your drivers
are good then it could be a hardware issue such as what I described above. If that's the case the fix is to replace the motherboard. Is the laptop still under warranty? If so, I would contact ASUS about it.
Another possible solution is to get an external USB sound device which is a PITA to use with a laptop.