Since 0.40 Incorrect audio output on MacOS when using Aggregate Audio Devices

mpv Information

mpv v0.40.0 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
libplacebo version: v7.349.0
FFmpeg version: 7.1.1
FFmpeg library versions:
   libavcodec      61.19.101
   libavdevice     61.3.100
   libavfilter     10.4.100
   libavformat     61.7.100
   libavutil       59.39.100
   libswresample   5.3.100
   libswscale      8.3.100

Other Information

- macOS version: macOS 15.3.2 (24D81)
- Source of mpv: https://formulae.brew.sh/formula/mpv
- Latest known working version: 0.39.x
- Issue started after the following happened: After update to 0.40

Reproduction Steps

After updating to mpv 0.40, previously working output to aggregate audio devices stopped working correctly.

I am using an aggregate audio device with a single multi-channel audio interface so that I can use my speakers, which are on different outputs (3,4) than the headphones (1,2), by just quickly changing audio devices.

This is accomplished by creating an aggregate audio device in MacOS's "Audio MIDI Setup" application, selecting the audio interface, and then clicking the Configure Speakers... button, and then setting the Configuration to Stereo and the Left and Right channels to the appropriate interface outputs.

I then set this new aggregate device as the default output device for MacOS

My mpv config includes audio-channels=stereo to ensure I only ever output to two channels. The issue occurs without this as well.

Expected Behavior

I expect that mpv will output audio on the correct outputs, like other applications.

Actual Behavior

mpv shows in the info OSD that it is outputting to a single (1) channel, and I hear audio in the left channel for my headphones (the first channel on the audio interface)

The issue did not occur before 0.40

Log File

output.txt

Sample Files

24bittest.flac.zip

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 reviewing output.txt and reproducing the issue with 24bittest.flac on macOS 15.3.2, using the aggregate device and the reported audio-channels settings. Trace the macOS audio output behavior introduced in mpv 0.40 and verify that stereo audio reaches the configured interface outputs rather than being reported as one channel.