Riddle Joker has an issue with Steam Play / Proton where it cannot Play Video files present in Riddle Joker (and various other titles)
This guide will fix that issue.
Step 1. Install ProtonUp-QT
Look for the Discover Store, It should be on the Taskbar by Default
Once you have opened it Search for “ProtonUp”
There should be a result for ProtonUp-QT
Click on it, then Click Install
Step 2. Install GE-Proton
Click “Add version”
Make sure its on GE-Proton, Now Click Install
Once the Install is finished GE-Proton will appear in the list of the App
You may now return to the Gaming Mode on Steam Deck
Step 3. Use GE-Proton
“Force the use of a Specific Steam Play compatibility tool”
GE-Proton
Once that is done you can back out of that section
Step 4. Play
It will take some time to open using GE-Proton for the first time.
Once it is done you are all set!
No launch commands or heavy tinkering required.
If you have completed a character route, to verify if it works, try going into Extra -> Scene Replay -> Movie, then click on any available video; if it plays, then it works.
Please leave a like if this worked for you!
(Credit to CreeperofDoom & R1kaB3rN for providing the fix)
Thanks to Creeper of Doom for their excellent guide; all credit belongs to their effort. If this guide helps you, please support and rate it here. Enjoy the game.