It is indeed possible to have independent circuits fail on your motherboard and still be able to turn on the motherboard; in fact, you're likely experiencing that now.
Fortunately, the two on board devices are not crucial to powering up. Whether the problem is 'common' in ASUS is beyond me, but I have had my share of difficulties with that brand.
To fix your problems, you could buy and install add-on PCI cards; one for LAN, and one for Audio/Sound. Alternatively, if the warranty hasn't expired, you could seek an RMA from Asus.