why do i get no response from audio input socket? have tried 4 different microphones in Final Cut Pro

I have started working on FInal Cut Pro and have discovered that my audio input socket does not recognise any microphone I plug in. I have tried 4 different microphones so far. THe input fader is turned up to 50% and beyond with no effect. The Input pane on the 'Audio' preferences just says 'Line Input' with no other device offered. Am I missing something here? It is 4 years old and this is the first time I have tried to input audio.