This was most likely due to the feature initially either having some unintended consequences, or being buggy on some devices.
Whatever the issues, it is most likely fixed now. Microsoft has pushed out an update to the OneNote app on Windows 10 PC and Mobile devices to reinstate the feature.
We should note that while OneNote Mobile received an update on both PCs and Mobile, the audio recording feature was only exposed on mobile devices. Microsoft presumably intends to bring that feature to Windows 10 PCs and tablets in the future, but today is not that day.
Download OneNote Mobile for Windows 10 here;