There seems to be a fairly common history of mic/audio problem with Skype on Macs. I've googled extensively and sifted thru this community but still have no answers.
I have a 2007 Intel Core 2 Duo MacBook Pro which originally ran 10.4.11. Whatever version of Skype should have worked with that was lacking in one area. The person I was Skyping with could barely hear me. That was the case with a couple of different people and they had successfully Skyped with others. Which points back to me. Yes, I've checked all my settings. I can make a Yahoo Msgr call with no issue, so it seems to be specific to Skype.
I had read that upgrading to newer Skype would resolve it. That involved me upgrading my Mac OS. So, I finally have moved up to 10.6.8 and downloaded the latest Skype for Mac, v. 18.104.22.1683 Problem solved? NOPE! Same exact problem. I can do a Yahoo Msgr call without issue, but not Skype. The video works on both ends for Skype and whomever I Skype with comes thru nice and clear on my speakers, but they can barely even tell I'm talking. Even when doing a test call, I could BARELY hear my own playback.
I've tried a headset that plugs into the speaker and mic jack and I've tried the external, built in mic. I switch my setting accordingly.
I have the same problem and I have tried everything that you have mentioned plus adjusting the audio MIDI settings under utilities and nothing. I'm so frustrated that I can't communicate with my friends and family because they can barely hear me when I can hear them fine. What's going on?