I decided to try playing U9 again, but when I started it up, the mouse sensitivity was low, even though it was fine last time I played, and I haven't touched the sensitivity settings of my touchpad. What do I need to edit to increase the sensitivity? I'm not using the Gog release, if that affects anything (it's fully patched, though).

Press Shift+Alt+M to increase the mouse sensivity. With Shift+Ctrl+M you can decrease the sensivity.