Summary
Armored Core VI crashes consistently on launch (or shortly after going fullscreen) with a Dantelion2 engine panic in the Wwise-based sound manager, followed by an access violation at a fixed instruction address. The crash is 100% reproducible and identical across every environment variable I've tested.

Crash signature

[Dantelion2 Panic]
....\Source\MOSoundManagerImpl.cpp(1120)
"m_pLowLevelBlockingIO is nullptr..."

Unhandled page fault on read access to 0x0000000000000018 at address 0000000142E56E15
EXCEPTION_ACCESS_VIOLATION exception (code=c0000005) raised

Same faulting address (0x142E56E15) every single run, regardless of the changes below.

System

Fedora 44, kernel 7.1.5-201.fc44.x86_64
GPU: NVIDIA Quadro RTX 3000 (laptop/Optimus hybrid with Intel iGPU)
Dual ALSA audio devices normally present: HDA Intel PCH (onboard) + HDA NVidia (GPU HDMI/DP audio, no display connected to it)

Things ruled out (identical crash under all of these):

Proton 9.0-4 (stock)
Proton 11.0-100 (stock)
GE-Proton 10-34
NVIDIA driver 595.80, 610.43.03, 610.57.04
Fresh Proton prefix (deleted and rebuilt)
Steam file integrity verify (clean)
X11 forced via SDL_VIDEODRIVER=x11
env --unset SDL_VIDEODRIVER
Wine's Pulse driver fully disabled (WINEDLLOVERRIDES="winepulse.drv=")
Wine forced to ALSA (WINEDLLOVERRIDES="winepulse.drv=n,b")
PULSE_LATENCY_MSEC=60
NVIDIA HDA audio codec disabled via snd-hda-intel enable=1,0 kernel module option (only one ALSA card present, confirmed via /proc/asound/cards)
PROTON_ENABLE_NVAPI=0
Explicit NVIDIA GPU offload (__NV_PRIME_RENDER_OFFLOAD=1 __VK_LAYER_NV_optimus=NVIDIA_only __GLX_VENDOR_LIBRARY_NAME=nvidia)
EAC disabled entirely (swapped start_protected_game.exe for base binary) — same crash without EAC in play

Notable log context immediately preceding crash (one run):

NvAPI_Disp_GetHdrCapabilities: Invalid argument

(No HDR display/setting present on this system; unclear if related.)

Similar report: A near-identical crash signature in the same engine (Elden Ring) with the same dual Intel+NVIDIA audio device topology was reported here and never resolved: https://bbs.archlinux.org/viewtopic.php?id=313754

Full PROTON_LOG output attached.

ac6-crash-log.txt