Audio stops working after...

Audio stops working after disconnecting external audio card.
I have a USB audio card attached to my USB on my desk, so when i dock my laptop i only need to plug in a USB to use my speakers (instead of 3.5mm jack as well). But sometimes when i plug in or unplug the audio device, the audio on my mac stops working. I cant change the volume (just get a cross over the icon), and if i try to open sound preferences, the window crashes. iTunes just doesn't play (stops on 0:00).

To resolve i have to restart.
The problem doesn't occur every time.