Q: My 2 month old Mac mini audio in port doesn't recognise the microphone on my webcam or a headphone microphone or a separate microphone all input via 3.5mm jack plugs - is there an off/on selector somewhere - or does it look like my audio in port is dead?

System Preferences > Sound > Input > Line In

Another thought..... because you are running Mac OS 9.0.x,  you may need to add a driver to access the external microphone.  Did any software come with your webcam, mic, or headphones? 

For example, although a bit foggy about the OS 9 days, I seem to recall that I had to add the included driver in order for my old Logitech USB Desktop Mic to work before OS X 10.2.8 and later made the mic compatible.

Message was edited by: EZ Jim

Mac OSX 10.9

The warranty entitles you to complimentary phone support for the first 90 days of ownership.