This has been an issue with some model PC's. When I was doing support on the Dell forums a couple of years ago there were a couple of Dell models that did not have the System Mixer. People tried various non-Dell drivers
but non-Dell drivers either did not install, did not work or would not work properly.
You can try other drivers, as I don't know if ASUS systems "must have" ASUS sound drivers, like the Dell's. If you can't find a driver
solution, there are some (paid for) software solutions that will function like the system mixer and allow you to record, for example, streaming audio from the internet. I don't recall what the program's are but I remember seeing one referenced in a post a couple of months ago. There was "Total Recorder" that did that function in Windows XP, I don't know if its been updated to work in Vista/Win 7 but you can check it out as a start.