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As soon as this happens, open Console and look for a recent error about IOUSBHostHIDDevice
My USB audio system disconnects from my Mac
QSince upgrading it to El Capitan, my iMac switches its audio output spontaneously, from the Bose Companion 5 speakers connected to its USB port back to its internal speakers. I have to restart the Mac or disconnect and reconnect the USB cable in order to continue using the external speakers, although sometimes they reconnect later. My Mac’s hardware has been checked, and is healthy. How can I keep my sound system working?
ASimilar problems have been reported with several other USB audio devices in El Capitan, so it seems to be a bug that hasn’t yet been fixed. It appears to occur because the Mac loses contact with the speakers, assumes that they have been disconnected, and then reverts to normal internal audio output.
You might notice a log entry when this occurs. As soon as it happens, open Console and look for a recent error message involving IOUSBHostHIDDevice, which would confirm it.
The best workaround at present seems to be to open Audio MIDI Setup tool, check your USB audio codec, which is responsible for this output, and set it to ‘44100.0 Hz’ and ‘2ch16bit Integer’ output. Some users report that it helps to leave this app open all the time.
It’s worth combing through your Library folders to root out any old extensions or audio plug-ins that may have been migrated from older software, as one could be cutting in and messing up settings. These can be blocked for testing by using safe mode: hold ß at the startup sound till you see the Apple logo.
Some people find their USB-connected audio devices, like these Bose speakers, spontaneously disconnect every so often under El Capitan.
The Audio MIDI Setup app, which is bundled with OS X in /Applications/Utilities, may be able to overcome glitches in which external audio devices spontaneously disconnect.