Take a look in Reliability History and Event Viewer.
Either one or both tools may be capturing some error event, warning, or even an informational event that occurs just before or at the time the microphone stops receiving input.
Reliability History provides a timeline format that may show the problem or some pattern of events that leads to the problem. User friendly.
Event Viewer requires more time and effort to understand.
To help:
http://www.tomshardware.com/faq/id-3128616/windows-event-viewer.html
Take your time with Event Viewer - no need to rush through it.
In both tools you can click any given entry to learn more about the error. Details may or may not be helpful.
Likely some app is getting launched as you cycle through alt-tabs. Or one of those apps, even if not the culprit, may be causing something else to happen.
Perhaps triggering another app (via Task Scheduler) to launch and that app, unknown to you, gains control of of microphone input. Which may, for that app, be disabled.