I am having this issue as well. Bug report on steams proton github is here https://github.com/ValveSoftware/Proton/issues/4102#issuecomment-1133888246
The error is as follows
[0522/091018.734:ERROR:mf_helpers.cc(12)] Error in dxva_video_decode_accelerator_win.cc on line 355
Unhandled Exception:
System.TypeLoadException: Could not load type of field 'SpeechSynthWrapper:_synthesizer' (4) due to: Could not load file or assembly 'System.Speech, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.
at (wrapper native-to-managed) <Module>.CreateSpeechSynthesizer()
[ERROR] FATAL UNHANDLED EXCEPTION: System.TypeLoadException: Could not load type of field 'SpeechSynthWrapper:_synthesizer' (4) due to: Could not load file or assembly 'System.Speech, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.
I know Linux is not supported but if you could provide any insight into the issue in the comments of the Proton issue tracker for Grounded, that would be greatly appreciated. Alternatively, if changes can be made in an update to the game, could you catch this exception and allow this subsystem to remain unloaded for systems that do not support Microsoft's speech synthesis engine?
