Audio drops out during playback of source clip AND in timeline

Hello,

Quite a frustrating problem, I hope someone here can point out an obvious solution.

I'm using a simple set up. Adobe Premiere 12.1.1 on a Macbook Pro running MacOS 10.13.3 with 8GB of memory. Videos are on an external hard drive, the caches are stored on the local drive. 

Now for the problem: The sound drops when I'm playing back a clip in preview window and in the timeline -- and it doesn't come back until I re-launch Adobe Premiere. I have cleared the cache. 

When I open the project file, I can play it. I'll hear it in the Preview Source window and then in the timeline. Then suddenly, the audio will drop. The audio meter reveals no sound to playback.

I had this problem months ago, and it suddenly went away and I haven't faced it for previous projects. I'm not the brightest tech guy, and other forums have not really answered the question / have had dramatically more complicated set ups.

Thank you for your help!