there is no known fix - all you can do is hope that the problem will be addressed in the next version of NewDark/SS2 2.4x. the game should be completable, but you will have to be really patient (and save a LOT). multiplayer only works up to three players, and currently, it's far from perfect - expect a lot of stability and synchronization issues. a lot of extended functionality is explained there. the SystemShock org troubleshooting guide [is also very useful, if maybe a bit technical. the GOG FAQ [is applicable to the steam version, check the useful links for solutions to some common issues.

See this very nice steam guide that will show you how to move the game properly (the mod part is outdated though, as mentioned before, using the modmanager [is the way to go). Running the game from Program Files can result in a wide variety of strange issues - crashes, missing audio, inability to save or change configuration, etc. the most important rule of a healthy SS2 install: move the game out of Program Files (unfortunately, this is the default steam location, so the chances your game is placed there after you download it are pretty high).

if you want to install mods, you should use a proper modmanager ( see the mod guide for more details and recommended mods [this should help you avoid problems with priorities and overwriting. if you drop your mods directly to the SS2 folder, you will break your game. it contains the updated engine and all the major graphics mods, so if it runs fine, you shouldn't have any problems with the full game either. there is a free demo available [you can try it out before buying the full game. if that won't help, try to patch up with ss2tool might help as well (do note that the modding part is outdated, go here [you want to mod your game). if the game refuses to start (or displays other odd behavior), uninstall it, delete everything from your SS2 folder except your saves, and reinstall it. Go here [apply all if you want to go as HD as possible, or stick just with SCP and SHTUP-ND if you want to stay close to vanilla.

How to download system shock 2 mods