You probably have one or several of problems. First make sure your Microphone is working by using your System Preferences->Sound and going to the Input tab. Just remember Macs do NOT have Microphone port, they have a audio Line-In port. To make a long story short, Microphone ports have power behind them so if a user wants to record audio in ( i.e. turntables, mini disk players, etc). Audio-In ports have no power behind it so no noise will be on the port when recording.
Second, a firewall(s) is blocking the signal. Here is an Apple document on "Well Known" TCP and UDP Ports Used By Apple Software Products". This will help if you or the distant end is having a problem.
Mac Pro Dual 2.8 Quad (2nd gen), 14G Ram, Two DVD-RW Drives, OS X 10.8.3
2006 Mac Book Pro 2.16 (first Gen) OS X 10.7.4
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32G iPhone 4S Black, iPad (3rd Gen) 32G Black