So the utility that came with the system? If not I'd check Asus's website (which you probably have) for your model and try to figure out what they use for a power management thingie for that system.
However in the Windows go to Start > Control Panel > Power Options.
There you should see an option for "High Performance". If not you may need to click a button with an arrow button beside where it says "Show Additional Plans". Switch to High Performance and it SHOULD keep everything important running when the laptop is plugged in (but will use power saving features when on battery).
From there (beside the option you have chosen) you should see "Change Plan Settings". If you click that you can do some minor changes BUT if you click Change Advanced Power Settings on that page then you get at the finite controls. There is an entry for Hard Disk which is pertinent here (as well as USB, CPU, etc). I turn ALL settings to be MORE that I expect to be using the system for. That means the screen (even DIM can potentially screw things up), disks, USB... anything in there at ALL. ALL OF IT! You never know what might make the audio skip or something screwup.
It does seem like you've already found all that though.
In the sidebar of the Power Options page (which you would be on if you see the stuff I described above) there should be an option called "Create A Power Plan" which will allow you to create a new power plan that will be saved and remembered by Windows and will appear in the list as Custom Plan alongside the Balanced and Performance plans. You can then switch to that whenever you want.
So if you don't want those hyper settings I just described you can create a new plan for audio then switch back and forth between plans as needed.
Also... you aren't using a USB hub or anything on this system are you?
Cheers.