According to Microsoft, this a side effect of a change it implemented to improve compatibility.
"At the request of some of our audio partners, we implemented a compatibility change that enabled certain games to query support and render multi-channel audio. Due to customer feedback, we are reverting this change as some games and some devices are not rendering multi-channel audio as expected. This may result in games sounding different than customers are used to and may have missing channels," Microsoft said.
"To mitigate the issue, open settings in the impacted game and disable multi-channel audio, if this option is available. You can also search in the Windows Control Panel for third-party audio device control panels and disable Multi-channel audio or Virtual Surround Sound, if these options are available," Microsoft says.
Whether the workaround actually works will depend on the specific game, as not all of them have the settings Microsoft mentions. In those cases, you may want to fiddle with your onboard or discrete sound card's settings. Otherwise, you'll have to play the waiting game.