How to fix my USB headset microphone (works for one second then imput stops working)

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http://askubuntu.com – I am using a Plantronics Headset via USB. This worked flawless for a long time. Now the microphone works for about 1 to 2 seconds when plugged in and then stops working. The hardware is alright (tested on other systems). Since this happens with Gnome or KDE alike I think there must be some flaw in the setup somewhere. Goole couldn't help - so now I am looking for some help to cleanly set up the whole USB soundsystem. How do I have to proceed to make sure the USB sound configuration is correct? (HowTos)