I have had this annoying problem ever since I built my new PC. The solutions you mentioned are what I've been doing to correct it. However, this makes volume adjusting very annoying.

When the sliders in the volume control are at a suitable level to minimize the internal sounds, it means I get an extremely small amount of travel on the speaker volume control. I can only turn it about 10-15 degrees.

I wish there were another way around this.

ps-oh, and I get this to a smaller extent using a PCI sound card as well. it was more noticeable when I was using onboard audio, but it's still there.