Audio Mixers For PCs?

ps writes: "Like many people, I work in a cubicle. I am also surrounded by several computers each with their own speakers and sound cards. And people complain that they are too loud (the volume won't go any lower!) I would love to have a simple "mixer" that I could wire in all my sound cards to one set of headphones. Nothing fancy, I just want to hear the alerts on all my boxes. Any ideas? I don't want to spend a fortune on this..." As michael puts it, like an KVM switch, but for audio.