There's also bug 1507182 with CDeviceEnumerator related crashes, but in DestroyHWndNotificationThread and with more useful stacks.
Of the 20 or so crash stacks I looked at, only one seemed to have any media/playback related threads around. Quite a few had a PlayEventSound thread from the widget code, but that code looks fairly simple and may not be implicated. Possibly crashing during audio init/deinit, maybe racy?
The crashes seem to be a mix of content process and unclassified ("process type" field blank, non-e10s?).
We can register an IMMNotificationClient (which has a OnPropertyValueChanged callback) from dom/media/AudioNotificationSender.cpp, dom/plugins/ipc/PluginUtilsWin.cpp, and two places in cubeb: wasapi_collection_notification_client which only landed in 67 via bug 1527659, and wasapi_endpoint_notification_client which we stopped registering in content processes back in 61 via bug 1427011 to avoid mystery crashes in CAudioSessionControl::QueueStreamSwitch.
Not much to go on here, but based on the above perhaps we should focus on the two AudioNotification cases in Gecko...