Windows 11's September preview update breaks AC-3 audio in older apps

Microsoft has confirmed a fault in KB5124010, the optional Windows 11 preview update released on 22 September, that can stop applications using AC-3 (Dolby Digital) audio decoding from working. The company's own notes say affected programs might fail to launch or close without warning.

What Microsoft says happens

The support article for the update, which covers OS builds 26300.9550, 26200.9550 and 26100.9550, lists the problem under known issues as applications that require AC-3 audio decoding closing unexpectedly. In an update to the Windows release health dashboard, Microsoft widened the description: in some cases the application can fail to launch, or close without warning, while in others it works normally until a feature is used, such as playing music. Affected software may vary and can include games, media players and certain productivity applications.

Legacy decoders, modern applications

Microsoft places the fault with Windows rather than with the applications. Programs that lean on the built-in AC-3 codec are the ones at risk, and the company describes the affected set as mostly legacy software; most modern applications ship their own audio decoding and are unaffected. There is no fix yet. Microsoft says it is working on a resolution for a future Windows update and will publish more information when it has it.

An optional update, except on some machines

KB5124010 is a preview update, the kind Microsoft ships in the fourth week of a month for testing, and it normally needs the user to click Download and install. That should limit how many machines pick it up, but the update installs automatically on Windows 11 24H2 devices where the option to get the latest updates as soon as they are available is switched on, a setting Microsoft nudges users towards. September's updates have already produced a cluster of problems: KB5124008 and KB5124012 needed emergency fixes for USB audio, Remote Desktop Services and Hyper-V faults, and the release health notes describe a separate credential problem from the same month.

Our opinion

The telling detail is not that a preview update broke audio decoding, because that is what preview updates are for. It is which applications break. The AC-3 path Microsoft is describing is the one old software uses because it never had to think about audio decoding at all, and the games and media players that still depend on it are frequently the ones nobody is maintaining. Those are also the programs least likely to publish their own workaround, and their users are the least likely to be reading a release health dashboard. Microsoft's mitigation is a settings toggle and a promise, which leaves anyone with that toggle switched on running a build that carries a known audio fault and no date for the fix.

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