[Regression] Proton Experimental/11.x: PROTON_PREFER_SDL=1 fails to enumerate DualSense & breaks audio in "The Saboteur" (Error 1726 & DSOUND 8-channel)

Issue transferred from https://github.com/ValveSoftware/Proton/issues/10021.
@mbtns posted on 2026-07-31T08:42:33:

Compatibility Report

• Name of the game with compatibility issues: The Saboteur / Vampire: The Masquerade – Swansong
• Steam AppID of the game: 24880 / 1299510

System Information

• GPU: Intel Arc A770 (DG2) / Intel UHD 770 (iGPU)
• Video driver version: Mesa (i915 driver) on NixOS 26.11
• Kernel version: Linux 6.x (NixOS 26.11 Zokor)
• Distro: NixOS 26.11 (Zokor)
• Link to full system information report as Gist
• Proton version: Proton Experimental / GE-Proton11-2 / GE-Proton11-3

I confirm:

• [x] that I haven't found an existing compatibility report for this game.
• [x] that I have checked whether there are updates for my system available.
• [x] that I have verified the problem happens on both Proton Experimental and GE-Proton 11-2/11-3.
• [x] that I am NOT using the GE-Proton flatpak.

Symptoms

1. DualSense (Wired) Not Recognized: When PROTON_PREFER_SDL=1 is set, the controller is completely invisible to the game in Proton Experimental and GE-Proton 11-2/11-3.
2. Audio Failure (The Saboteur): Complete silence.
3. Critical Log Errors:
   • err:service:device_notify_proc failed to get event, error 1726 (RPC failure preventing device enumeration).
   • fixme:dsound:DSOUND_MixerVol There is no support for 8 channels (Audio stack failure).
4. Regression: These issues are absent in GE-Proton11-1 (which works perfectly).

Reproduction

1. Launch The Saboteur (AppID: 24880) or Vampire: The Masquerade – Swansong (AppID: 1299510) on NixOS.
2. Select Proton Experimental or GE-Proton11-2/11-3.
3. Set launch option: PROTON_PREFER_SDL=1.
4. Connect DualSense via USB.
5. Result: Game launches, but controller is dead and audio is silent (Saboteur). Log shows error 1726.
6. Comparison: Switch to GE-Proton11-1 -> Controller and Audio work immediately.

Log Excerpts

01fc:fixme:dsound:DSOUND_MixerVol There is no support for 8 channels
015c:err:service:device_notify_proc failed to get event, error 1726
gamemodeauto: dlopen failed - libgamemode.so: cannot open shared object file: No such file or directory
0408:fixme:winediag:loader_init wine-staging 11.0 is a testing version containing experimental patches.
Launch command: ['/usr/bin/gamemoderun', '/usr/bin/umu-run', '/home/batenssegher/Games/Heroic/Games/The Saboteur/Saboteur.exe']