So I've actually had this problem for over half a year but haven't gotten around to fixing it, yet. First some history on my laptop:

I've had it for about 1 and a half years now, so I'd call it quite new. When I first installed linux and was using the onboard sound (Intel ICH7) I noticed some strange things. First, there was always this low humming sound. I didn't really hear it when watching a movie or listening to music but in pauses I could often hear it. But it was a lot stranger. I noticed that the sound got a lot louder when I was moving around windows on my desktop. The bigger the window, the louder the sound. When I stopped moving anything, the sound got barely audible again. By the way my video card is an ATI Mobility Radeon 1400, in case that's important. I concluded that there had to be something wrong with my laptop's mainboard or chipset or something like that and that the problem was out of my control, so I chose to ignore it.

Now half a year ago I wanted surround sound. I borrowed my sister's soundcard (Terrated Aureon 5.1 USB MKII) and tried it out on linux. It seemed to work good, except for some crackling that I thought must be coincidence or somehow part of the song or maybe my ears playing tricks.. so I bought one for myself.

At first everything seemed to work fine, I could hear music on all speakers without any problems and the quality was really good. But then I wanted to watch a movie with surround sound and everything went bad:

When using Xine as video player, I would sometimes hear a crack and suddenly some of the channels/speakers would be exchanged. I think the center switches to rear-left and the other way around. And some others change as well. Then shortly afterwards there'd be another crack and it would be back to normal. When I watch the movie in a window instead of full screen the cracking happens all the time (like 10 times per second or so) and the channels are just switching around without end. The same happens when I play videos in MPlayer, no matter if it's full screen or not.

By now I know that whenever either XVideo or OpenGL is used, there'll be problems. If I play videos on X11 without any layers (mplayer -vo x11), the sound will work just fine, but I can't very well do that all the time cause the scaling will be too slow.

But it gets stranger still. As I said earlier, when watching movies in Xine, the problems are down to a minimum, I don't really notice it anymore. But I noticed when I watched a video several times, the cracking sound and the channel switching will always happen at the same moment. It's like there are scenes with problems and scenes without them, and generally the higher the resolution of the video the more problems there are.

I'm pretty sure the sound card is mostly innocent in all this. My sister's card, which is four years older than mine, makes the same problems (only when connected to my laptop of course) and I've read on the Internet that many people are using the card without any problems, also in Linux.
I don't know if my problem is Linux-related or not though, since I have no other Linux computer to test it with.

Does anyone know of such a problem? How can one piece of hardware (video card) influence another one (USB sound card). This problem just never ceases to puzzle me.
Since I've had problems with the onboard sound, too, my assumption is that are some voltages somewhere in the laptop (Asus A7CC) where it definitely shouldn't be and making problems for everything that's connected. Is that possible? Can it be fixed maybe?

I haven't had any problems with other pieces of hardware, yet. So far it's always been sound and video (I should mention that the laptop's analog video output looks kinda crappy, too. DVI is nice, though).

I noticed this behavior with my onboard Intel video on an ICH4 laptop. It only seems to happen, at least in my experience, when using either Generic VESA or Generic VGA driver. What driver is your computer set up to use right now? Which Linux distribution are you using? What kernel version are you running? Thanks!

I'm running Gentoo with kernel 2.6.22-suspend2-r2.

I'm using the proprietary ATI drivers for my video card and I think those are the only ones that work. I haven't updated the drivers since I got the sound card, but the onboard sound problems have remained all the time, even though I've updated the drivers several times.

The proprietary ATI drivers have many issues, though. They may even be the source of the problem. The X server sometimes crashes when running 3D apps (like Google Earth or screen savers) and in the current version of the drivers XVideo uses linear scaling so I have to use OpenGL for video playback... also the picture often gets those diagonal cuts when there is a lot of movement in a video. But I can't really imagine that the drivers can cause something like the problem I got there... I'm pretty sure it's something hardware related...