This problem is already solved, but it has troubled me across several games and in the interest of building up a lot of the historical gaming knowledge lost on forums and Reddit, I’d like to post here. I don’t fully understand the problem yet, so if you have more info to share, please post.

PROBLEM STATEMENT: If you’re playing an older Unity game on Steam under Linux (either on your Steam Deck or desktop) and experience black screens, errors, or crashes when a movie plays such as an opening cinematic, try re-encoding the videos with HandBrake. Credit to Bird Observer on the River City Girls discussions where I found this and generalized the instructions:

  1. From Desktop mode, right-click the game in your Steam library and select Manage > Browse local files
  2. Find the folder containing the video assets (OPTIONAL: copy the folder into a backup location to prevent having to redownload the files if you make a mistake)
  3. Start HandBrake and click Open Source, then navigate to the game folder(s) you discovered in Step 2 (download HandBrake from Discover or https://flathub.org/)
  4. Use Shift or Ctrl to select the movie assets
  5. Settings
    • Preset: Official > General > Fast 1080p30
    • Format: MPEG-4 (avformat)
    • Align A/V Start & Passthru Common Metadata ✅
  6. Set a destination folder under To: at the bottom of the HandBrake window, I recommend a separate working directory under ~/Videos or wherever
  7. From the top menu bar, click the dropdown arrow (v) next to Add To Queue and choose Add All
  8. From the top menu bar on the right, click Queue and then select Start. This can take several minutes to complete depending on your system and how many/how large the movie files are
  9. When finished, copy the completed files from your working directory back into the appropriate game directory

NOTE: For some games using .wmv, simply re-encoding them to .mp4 and then changing the file name back to .wmv should be sufficient for the game to find the appropriate file and play it without needing to worry about further encoding or format issues.

I hope that helps someone, and again, if you have any additional steps or information to help clarify the topic, please feel free to add! I suspect this is largely applicable to Unity games, but may help with other engines where the movie assets are unpacked and easily accessible.