Audio doesn't play (sometimes)

I have a course with audio.  Has been playing fine during preview and then when I publish it sometimes the audio plays and sometimes it doesn't.

Have checked all my sound / volume elements and all ok.

If I go into a slide and click the audio file, the sound options won't play it either.

I have to restart Storyline to get the audio to play.  Inconsistent - not happening all the time.  Any thoughts.

I am on a MAC using Parallels desktop and Storyline has been running fine.

Not sure what to look at next.

Thanks in advance