When I'm playing, when an area loads it will say "ERROR MESSAGE: MESSAGE DOES NOT SAY AN ERROR" or something like that. I assume this means it doesn't know what the problem is. Is this just a windows 7 issue?

it comes up when you have the midi selected, mute the midi in control panel & it goes.
check your midi settings, if it shows default microsoft midi the game will only crash later on anyway. this appeared after win 98.

Thanks. So does that mean there's no way to have the in-game music play?

you have music but not any background music, we have been playing overseer for years without any midi . I found my toshiba laptop has no sound card but it plays the midi.
depends on pc hardware on whether you can play the midi.

google vista midi mapper & install it & see what comes up on your pc. If it just shows the defalt microsoft one, just disable midi.