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