I can hear myself in Speakers

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http://askubuntu.com – im trying to set up my microphone to work with Ubuntu 12 The Problem is that i can hear myself when i speak. And if i play for example music, the music then triggers the indicator for the microphone volume. So the output of the speakers is somehow also send to the microphone. So it's impossible to use for example Teamspeak because of the rustle. How can I fix this? (HowTos)