I am seeing the following behavior on a MacBook Air and an iMac.
We have external speakers set up that we connect to via USB > external sound card > speakers. When we initially connect, the audio works as per normal, sounds great. Then after about a minute it mutes completely, and won't turn on unless we reconnect the usb and select it again as the output. It's not iTunes or Spotify because using the volume up and down keys does not make any sound either.
The iMac started doing this 4 or 5 months ago and now it has just started on the MacBook Air. This also happens on the MacBook Air when I try to connect audio out to the external display I have connected to it, so I don't think its the sound card, I think it might be OS software related.
Sound panel screenshot:
Some more info - when I switch between the two USB audio outputs in the sound panel, I get a short flicker of sound before it cuts out again.
Turning off Bluetooth and WiFi had no effect - the problem still persists