Matchbox pro 2010 i7 2.0, 8gig mem, never had this problem before. If I unplug USB after a few seconds it will function properly after it switches to built in audio.
I am using an AudioBox USB, not an i-Series. Owned this unit for at least 5 years. Tried both usb ports on my MacBook. Operating system is El Capitan. If I open iTunes and use the Audiobox USB as sound output, it will play through the monitors and then start to switch back and forth between the Audiobox and built in output.