Unfortunately you will just have to keep sending it in. You could try to ask for a replacement since they still havent fixed it but they dont have to do that. I'd suggest sending it in one last time and explain the problem in extensive detail so they know what exactly the problem is.
I know there have been problems with phones before where a software update has disabled the mic on the phone. You could try giving the phone a factory reset. Also does the mic work on other appications other than skype? If it does it could simply be a Skype problem.
If you dont want to send it in again you could just you a headset with a mic.