Remix: stereo -> 7.1 not working

mpv Information

[   0.002][v][cplayer] mpv v0.40.0-dev-g748260893 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
[   0.002][v][cplayer]  built on Nov 17 2025 04:53:58
[   0.002][v][cplayer] libplacebo version: v7.357.0
[   0.002][v][cplayer] FFmpeg version: e57d7aa
[   0.002][v][cplayer] FFmpeg library versions:
[   0.002][v][cplayer]    libavcodec      62.11.100
[   0.002][v][cplayer]    libavdevice     62.1.100
[   0.002][v][cplayer]    libavfilter     11.4.100
[   0.002][v][cplayer]    libavformat     62.3.100
[   0.002][v][cplayer]    libavutil       60.8.100
[   0.002][v][cplayer]    libswresample   6.1.100
[   0.002][v][cplayer]    libswscale      9.1.100
[   0.002][v][cplayer] Configuration: -Ddefault_library=static -Dlibmpv=true -Dtests=true -Dgpl=true -Dffmpeg:gpl=enabled -Dffmpeg:tests=enabled -Dffmpeg:programs=enabled -Dffmpeg:sdl2=disabled -Dffmpeg:vulkan=auto -Dffmpeg:libdav1d=enabled -Dffmpeg:libjxl=enabled -Dffmpeg:libaom=enabled -Dlcms2:fastfloat=true -Dlcms2:jpeg=disabled -Dlcms2:tiff=disabled -Dlibass:test=enabled -Dlibjpeg-turbo:tests=disabled -Dlibusb:tests=false -Dlibusb:examples=false -Dlibplacebo:demos=false -Dlibplacebo:lcms=enabled -Dlibplacebo:shaderc=enabled -Dlibplacebo:tests=false -Dlibplacebo:vulkan=enabled -Dlibplacebo:d3d11=enabled -Dxxhash:inline-all=true -Dxxhash:cli=false -Dluajit:amalgam=true -Dd3d11=enabled -Dvulkan=enabled -Djavascript=enabled -Dwin32-smtc=enabled -Dlua=luajit -Ddrm=disabled -Dlibarchive=disabled -Drubberband=disabled -Dwayland=disabled -Dx11=disabled -Dwrap_mode=forcefallback
[   0.002][v][cplayer] List of enabled features: build-date cplugins cuda-hwaccel cuda-interop d3d-hwaccel d3d11 d3d9-hwaccel debug direct3d dos-paths dxgi-debug-d3d11 ffmpeg ffnvcodec gl gl-win32 glob glob-win32 gpl iconv javascript jpeg lcms2 libass libavdevice libdl libplacebo luajit shaderc spirv-cross uchardet vector vk-khr-display vulkan wasapi win32 win32-desktop win32-executable win32-smtc win32-threads zlib

Other Information

- Windows version: Windows 11 25H2 
- GPU model, driver and version: AMD Ryzen 7 9800X3D
- Source of mpv: https://nightly.link/mpv-player/mpv/workflows/build/master/mpv-x86_64-pc-windows-msvc
- Latest known working version: -
- Issue started after the following happened: -

Reproduction Steps

I am using: "audio-channels=8" via config. The log shows "Remix: stereo -> 7.1" but I only hear sound from the front left/right speakers. Log attached.

Expected Behavior

Sounds plays 7 speakers + sub

Actual Behavior

Only two speakers have sound

Log File

logv.txt

Sample Files

Any stereo mp3

I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Start by reproducing the issue with audio-channels=8 and an ordinary stereo MP3, then inspect the attached logv.txt alongside mpv's audio remix and output handling. Done means stereo playback is routed to all seven speakers and the subwoofer on the reported Windows setup, with a regression test or other verification if the relevant test location is identified.