When i install pclos-KDE on my Aspire D255E, Kmix don't show the mics pots by default, you have to activate them, if you need them. They are named MIC and MIC BOOST. I suppose MIC BOOST is for the external jack. This is for KDE; On XFCE, i have no idea.
Also, do your friends a favor: Use a mic closer to your mouth instead of the internal mic, Reverberation is the enemy of intelligibility.