As we all know, skype>ubuntu>pulseaudio usually have some issues that causes mic problems.
I think i have solved that issue on my natty by installing alsamixer packages and deleted full Pulseaudio package.
Of course, the Banshee connection on the top bar will disappear, because we have to delete "Ubuntu-Desktop" package as well, but for me was the only way to solve it. Now, i use alsamixer controllers on the left unity bar to control volume.
Don't forget to change de soundboard on skype options (choose this option if this is your case: "HDA ATI SB ALC888 Analog Default Audio Device")
and in the terminal (CTRL+ALT+T) type: alsamixer
then you'll have the power to control all the sound of your machine and perhaps you should take care about disable "mic boost" and enable "Capture".
Do you agree with this? I should remember my computer definitions:
Acer Aspire 5542G
AMD TurionII X2 M500
ATI Mobility Radeon HD 4570
I hope i helped you.