no mic and "Audio timestamp for device ... was behind target timestamp" also happens to me:
1. always when I set scene buffering time to 200 ms and mic offset to -200.
2. rarely when my CPU is 100% used up.
3. sometimes when I run OBS from visual studio 2013 debugger, record, pause (breakpoint), wait 20 seconds, then resume. I mostly don't use debugger when live streaming.
In all cases, "reinitialize" button doesn't bring microphone back up and working in OBS. stop record and stop stream does.
I use VB-Audio Virtual Cable as a mic input in OBS, which appear to reduce chances of obs's mic audio cutting out.