Please try to use the Knightmare unofficial patch 1.42D for the Return to Castle Wolfenstein. You can download it from here:
After downloading the archive, extract it's content to game's installation folder, overwriting existing files.
In order to navigate to game's installation folder:
- If the game was installed via Galaxy, please select the game in Galaxy, click on More -> Manage Installation -> Show Folder
- If the game was installed via offline installer, then you will have to manually navigate to the game installation folder, by default it's: C:\GOG Games\Return to Castle Wolfenstein\
Once that is done, run the game using WolfSP executable.
Alternatively you might want to try IORCTW. You can download latest versions of it from here:
Once downloaded, copy the content of iortcw folder (folders Docs, Main, and all files) to game's installation folder as instructed above.
Make sure to run the game using ioWolfSP executable.