Can you hear anything. iTunes, other sounds? Have you tried different people. There are 2 things I thought of for this problem. 1, the other person is the same person all the time and they have muted their microphone. 2, you cannot hear anything on your system which could be mute, or a driver problem or a problem that can sometimes occur with the headphone jack port.
For #1, have the other person try to record something or go to System Preferences and select sound, and make sure the microphone shows gain when they talk, and make sure the microphone is turned up (see picture for this one.) #2 You can try to unmute in the same system preferences, you can also check to make sure your sound system is working in the same preferences. If it is a headphone jack issue, I would plug in and out head phones into the jack until you can hear something (make sure you can hear through the headphones before this, if the headphones give no sound then don't do this.)
Other than those items it would have to be a call apple issue.