Gaming system with laptop as monitor/keyboard

The sound isn't the problem. Well, not any more than the rest of it. The problem is that laptops rarely have inputs for external devices (like a graphics card to send images to the screen), and have even fewer outputs for their built-in components, like the keyboard.

What you would need to do is somehow mod the keyboard's output into a USB or PS/2 connector and run that to the new build's case. Then you would need to run the video display from the computer to the laptop and find some way to hack the current input, assuming that you've got a discrete video card on the laptop. If your laptop uses integrated graphics, it would be even more difficult.

Good 1080p monitors can be had for between $150-200, and keyboards start at about $10. The work required for modding it isn't that extreme...if you already know how to do it. If you don't know how to do the modding, I'd guess you're looking at 10+ hours to get it figured out.