I'm not very tech savvy so bear that in mind. It's a Mac Mini and it's connected by HDMI cable to the receiver I use for surround sound. I'm running Spotify, YouTube etc on the machine without any sound problems.

There's a new piece of info - if I click on the audio output in the bottom left hand corner, there are 3 options - HDMI/Built in Output/Instant On Sound Effects. When I open the program it (correctly) shows HDMI as selected. If the sound stops working I seem to be able to get it back by clicking on one of the other options and then clicking on HDMI again. This is less hassle than shutting down and re-opening the program so a step forward, but of course it would be better still if the sound didn't disappear in the first place.