This is a recent problem and I'm not sure if it is the result of a software update but it has become a real problem for our work flow.
When recording narration (or individual slide audio) an audio track longer than 40 seconds is either cut off mid flow or becomes garbled and speeds to the end of the section. With the latter the audio track appears to be the appropriate length but as it reaches the 40 second mark just speeds up and becomes incomprehensible.
I've seen similar posts appearing on the support pages and Microsoft has put it down to a corrupted PPT(X) file. I've tried this with old files and new files across multiple PCs with different individuals logged in. I've tried USB headsets and those with audio jacks. All have the same issue.
I thought it might be a buffering issue as a result of the length of the narrated presentation even though this has not been a problem in the past. When I tried recording individual audio tracks rather than recording narration across the entire presentation I get the same result.