Недавно решал такую проблему для человека нашел небольшое HowTo на английском, думаю все будет понятно
Have a try - just to be sure:
to the right of the top panel there is a volume control applet - it looks like a speaker.
right-click on it, and select "Open Volume Control"
from the window that opens,
select "change device" from the file menu. Make sure the "alsa" device is selected.
select "preferences" from the edit menu, and add useful looking controls.
In the playback tab, make sure the microphone is not muted. (There may be several.)
In switches tab, enable the mic boost.
In system > preferences > sound - audio conferencing, the capture device must match the mic device. You can play with this - usually only one will work at all with the test.
In system > preferences > multimedia systems selector - fiddle with the input sound stuff.
(You may need to odd this entry to the main menu)
Hope you have better luck than I did.