Why does USB audio out mute after about 1 minute? 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:


Edit:

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.

Another edit: 

Turning off Bluetooth and WiFi had no effect - the problem still persists
I have problems with the Alesis M1Active 520USB speakers. After an undetermined amount of time they just stop working. Switching to internal speakers works fine. A reboot fixes it...but that's painful. The easier solution I've found is this:


Change your sound source (option clicking the sound icon is the quickest method) to Internal Speakers.
Unplug the speakers
Open a terminal and run this command, which restarts the Core Audio Daemon: sudo killall coreaudiod
Plug the speakers back in


Maybe it will help you too.