Hi,
I believe I have found a Core Audio regression in macOS 27. This exact hardware configuration worked correctly on macOS 26, and the issue started immediately after upgrading to macOS 27.
Hardware:
Mac mini M2
2 × MSI MP243X monitors
Monitor 1 connected via HDMI
Monitor 2 connected via DisplayPort
Both monitors expose a 2-channel audio output to macOS
On macOS 26:
I used Audio MIDI Setup to create a Multi-Output Device containing both MSI MP243X audio outputs. Audio played simultaneously through both monitors without any issues.
On macOS 27:
Both audio devices are still detected correctly and both work perfectly when selected individually:
HDMI only: WORKS
DisplayPort only: WORKS
HDMI + DisplayPort simultaneously: FAILS
This is reproducible even immediately after a full reboot.
When I create a new Multi-Output Device containing both MSI outputs and select it as the system output, audio playback cannot start.
For example:
afplay /System/Library/Sounds/Glass.aiff
returns:
Error: AudioQueueStart failed ('stop')
I captured Core Audio debug logs while reproducing the problem. Core Audio initially reports that StartIO succeeded, but the physical display audio device then fails to establish a timeline.
Relevant log messages include:
HALS_IOEngine2::_StartIO succeeded
Device 3669B540-0000-0000-2623-0104A5351D78 is not running.
could not establish a timeline after waiting 10000000 microseconds
stopping with error 1937010544
StartIOThread: the IO thread failed to start, Error: 1937010544
I have already tested the following:
Both devices at 44.1 kHz / 24-bit
Both devices at 48 kHz / 16-bit
Recreating the Multi-Output Device from scratch
Using the HDMI device as clock source
Using the DisplayPort device as clock source
Different drift correction configurations
Restarting coreaudiod
Full macOS reboot
Testing both outputs individually immediately after reboot
Alternative multi-device audio routing software
The behavior is always reproducible.
Each MSI MP243X audio endpoint works correctly by itself. The failure only occurs when both HDMI and DisplayPort audio endpoints are opened simultaneously.
system_profiler correctly detects both devices at 48 kHz:
MSI MP243X
Output Channels: 2
Current Sample Rate: 48000
Transport: HDMI
MSI MP243X
Output Channels: 2
Current Sample Rate: 48000
Transport: DisplayPort
This does not appear to be a problem with either monitor or connection individually, because both outputs continue to work independently.
The important difference is the macOS version:
macOS 26:
HDMI + DisplayPort Multi-Output = WORKING
macOS 27:
HDMI only = WORKING
DisplayPort only = WORKING
HDMI + DisplayPort Multi-Output = FAILS with AudioQueueStart 'stop' / Core Audio error 1937010544
No hardware, monitor, cable, or connection changes were made between the working macOS 26 configuration and the macOS 27 configuration.
This therefore appears to be a regression in the Core Audio/display-audio path introduced with macOS 27 when two display audio endpoints (HDMI + DisplayPort) are opened simultaneously.
Has anyone else been able to reproduce this on macOS 27, particularly on Apple Silicon?
I can provide the complete coreaudiod logs and run additional Core Audio diagnostics if needed.
Additional isolation tests:
Both monitors can remain physically connected to the Mac at the same time.
With both HDMI and DisplayPort connected:
Selecting the HDMI MSI MP243X as the only output works correctly.
Selecting the DisplayPort MSI MP243X as the only output works correctly.
Opening both audio endpoints simultaneously causes the failure.
Therefore, simply having both displays connected does not trigger the issue. The failure appears specifically when Core Audio attempts to run both display-audio endpoints at the same time.
I also tested this using third-party multi-device audio routing implementations instead of relying only on Audio MIDI Setup.
The same behavior occurs: either MSI MP243X can be opened and used independently, but attempting to route audio to both devices simultaneously fails.
This suggests that the issue is below the Multi-Output Device configuration itself and may be related to starting/synchronizing two display-audio IO engines simultaneously.
During the failure, the Core Audio log sequence is particularly interesting:
The physical device is detected normally.
The audio format is configured successfully.
Core Audio reports that StartIO succeeded.
The device subsequently reports that it is not running.
Core Audio waits 10 seconds for a timeline.
Timeline establishment fails.
The IO thread terminates with error 1937010544 ('stop').
One captured conversion path was:
2 ch, 48000 Hz, Float32, interleaved
->
2 ch, 48000 Hz, Int16, interleaved
so both sides were operating at 48 kHz during this test.
Another important observation is that restarting coreaudiod does not resolve the simultaneous-output problem.
A complete reboot also does not resolve it: immediately after a fresh boot, before launching any third-party audio-routing software, both physical outputs work individually but a newly created Multi-Output Device still reproduces the failure.
This makes the issue highly reproducible on this machine.
Minimal reproduction:
Boot macOS 27 with both MSI MP243X monitors connected.
Open Audio MIDI Setup.
Verify both MSI MP243X devices work independently.
Set both devices to 48,000 Hz.
Create a new Multi-Output Device.
Select both MSI MP243X devices.
Select the new Multi-Output Device as the system audio output.
Run:
afplay /System/Library/Sounds/Glass.aiff
Expected result:
The sound is reproduced simultaneously through both monitors, as it was on macOS 26.
Actual result:
Error: AudioQueueStart failed ('stop')
Switching the system output back to either individual MSI MP243X restores audio playback.
The problem can then be reproduced again by selecting the Multi-Output Device.
