msmpeg2ac3dec.dll. Two days after release, Microsoft added a third item to the release notes that is easy to miss and arguably the most important one: a new recovery tool called Cloud rebuild.Here is what is in the build, who gets it, and what it means for people outside the Insider program who are dealing with audio-related crashes.
The build at a glance
Microsoft's Windows Insider Blog announced on October 7, 2026 that it was releasing new Windows 11 Insider Preview builds to the Beta and Experimental channels. The lineup was Beta: Build 26220.9606 and Experimental: Build 26340.9616. The blog also said the release came straight off the back of Microsoft's Windows + Surface event. Neowin noted that people hoping for a "Windows 12" reveal at that event went home disappointed. It also reported that in the beta channels, Microsoft published build 26220.9606 for 25H2, and 28020.3172 for 26H1.
Microsoft's release notes put the build on the 25H2 line: updates are based on Windows 11, version 25H2 via an enablement package (Build 26220.9606). The release page also shows a later revision: release date 7 October 2026, updated 9 October 2026 with the addition of cloud rebuild in the notes.
| Item | Detail |
|---|---|
| Build | 26220.9606 |
| Channel | Beta |
| Base | Windows 11, version 25H2 (enablement package) |
| Released | October 7, 2026 |
| Notes revised | October 9, 2026 (Cloud rebuild added) |
| Sibling builds | Experimental 26340.9616; Beta (26H1) 28020.3172 |
Summary: This is a Beta Channel build for 25H2, not a public update. Microsoft added to the release notes after launch.
Weather takes over the lock screen
The most visible change is on the lock screen. Microsoft describes the update as part of its continued commitment to making Windows calmer and higher quality, updating the default lock screen widget experience for Insiders on newer builds. In practice, Weather will be the primary widget shown on the lock screen, reducing recommended widget content and making the experience feel more focused.
Users also get more control. Microsoft says it is giving users more control of personalizing the widgets on lock, curating the ones they find most useful for them.
To try it on a Beta build:
- Open Settings.
- Go to Personalization > Lock screen.
- Review the widget options there and keep the ones you actually look at.
Lock-screen widgets are not new. In August 2025, Microsoft's Release Preview notes for Build 26100.5061 described adding, removing and rearranging lock-screen widgets such as Weather, Watchlist, Sports and Traffic from the same Settings page. Any widget that supported the small size could be used. What changes in this build is the default: less recommended content, with Weather given top billing. The release notes do not list exactly which widgets can be chosen in 26220.9606, and they do not say how much recommended content was removed.
Is "calmer" a real quality gain or a marketing word? Windows users have long complained that widgets push news and recommendations where they never asked for them. A default that shows less of that content is a welcome change in direction. Still, "calmer and higher quality" is Microsoft's description, not a measured result.
Summary: The default lock screen now leads with Weather and shows less recommended content. Your choices live under Settings > Personalization > Lock screen.
The fix for msmpeg2ac3dec.dll crashes
The second change in the notes is a single line: Microsoft fixed an issue with the component msmpeg2ac3dec.dll that was causing some apps and games to crash in recent builds. Windows Report calls msmpeg2ac3dec.dll a Windows media component. It also notes that Microsoft did not provide a full list of affected software. The notes do not explain the cause either.
The likely link to retail Windows is a known issue on Microsoft's release health dashboard. It is a separate entry, and as Notebookcheck put it, Microsoft documents a confirmed known issue on its release health page. According to Microsoft's entry for Windows 11 24H2, 25H2 and 26H2:
- Trigger: Windows updates released on or after September 22, 2026 (KB5124010).
- Symptom: Some apps that need AC-3 (Dolby Digital) audio decoding may close unexpectedly. Some fail to launch. Others run normally until a feature such as music playback is used.
- Likely victims: Games, media players and some productivity apps. Microsoft says legacy apps that rely on Windows' built-in audio decoding are most at risk. Many modern apps include their own decoders and are unlikely to be affected.
- Scope: Windows 11 versions 24H2, 25H2 and 26H2. No Windows Server versions are listed.
- Status: Confirmed. Microsoft says it is working on a fix for a future Windows update, with no date given.
The component name, timing and symptoms all point to the same problem. However, Microsoft has not said the Beta fix resolves the release health issue, and we will not claim it does. Be careful with advice found elsewhere. Microsoft's entry offers no workaround. Downloading a replacement DLL from a random website to swap a system media component is a good way to turn one problem into several.
What to do:
- Beta Insiders: Install 26220.9606 and check whether apps that crashed before now work. If they don't, report it in Feedback Hub.
- Everyone else: Watch the release health entry and upcoming cumulative updates. If a legacy app or older game depends on Dolby Digital audio, check whether it has a newer version that ships its own decoder.
- IT admins: If help desk tickets mention old media apps closing after the September 22 update, this known issue is a strong lead.
Summary: The Beta build fixes msmpeg2ac3dec.dll crashes. A matching AC-3 issue is confirmed on retail Windows 11 but has no fix date yet.
Cloud rebuild: added to the notes after launch
The October 9 revision adds a recovery feature. Microsoft describes Cloud rebuild as a new recovery option that restores a Windows 11 PC to a clean, known-good state by performing a full OS reinstall, even when Windows won't boot. It differs from the existing reset in this way: unlike Reset this PC, Cloud rebuild downloads both the target Windows image and the device's drivers from Windows Update, so the device comes back fully functional without USB media, without a custom image, and without depending on the health of the currently installed OS.
Microsoft lists these steps for trying it:
- Start it directly from the Windows Recovery Environment (WinRE) by selecting Troubleshoot > Recovery and uninstall > Cloud rebuild.
- Connect over wired Ethernet or a Wi-Fi network from within WinRE.
- Review the target Windows build, edition, and language, then confirm the data-loss warning before the rebuild begins.
Take that data-loss warning seriously. This is a full reinstall, not a quick tune-up, so back up first. Microsoft asks for feedback in Feedback Hub under Recovery > Cloud rebuild.
Why does this matter? Anyone who has kept a "just in case" USB installer in a desk drawer knows the problem it solves. When the local install is too broken to repair itself, Reset this PC has little to work with. Pulling both the Windows image and the drivers from Windows Update removes that dependency. A few qualifications: the PC still has to reach WinRE and get online, and this is an Insider preview. Treat it as a promising test, not something to rely on yet.
Summary: Cloud rebuild reinstalls Windows using an image and drivers from Windows Update, starting in WinRE. It is a preview feature and it wipes data.
The usual Insider caveats
Microsoft notes that features in these builds may change, be removed, or never get released beyond Windows Insiders. Nothing here has a public release date. That includes the crash fix, which is the item retail users will care about most.
The other October 7 builds had different headline features. According to Microsoft's announcement, the new Battery Status widget went to the Experimental channel. A summary on daily.dev also notes a redesigned Windows Search experience built on WinUI 3 with inline quick actions. Beta users on 26220.9606 should not expect either one.
Bottom line
Build 26220.9606 is a small update. The lock screen gets less cluttered, a crash fix covers a component that is also causing documented problems on retail Windows 11, and Cloud rebuild arrives quietly through the October 9 notes revision. Insiders should test all three. Everyone else should watch the AC-3 entry on the release health dashboard, since that is where a fix for retail Windows 11 is most likely to be announced first.