Why is the input level of the external mic so low? I try to use an external mic to make some audio recordings. But the level of the input is really low ! In fact, the internal build-in mac has a better input level than the external one that is closer to my mouth !

Did I miss something about audio recording ?
Microphones are hooked up to a pre-amp to boost the signal before recording. The internal microphone on the Mac probably has a built-in pre-amp while the external jack doesn't. Typically external mikes are connected to a sound or mixing board before hooking up to a computer.