Plug headphone not support with macmini
Here are the specs for this mini according to Wikipedia. Note that this Mac (and most Macs I'm familiar with) has an audio line-in which is a high-level input, but no mic-in (low level). That means you'll need an external microphone amplifier between the mic and the line-in input. The USB mic is doing all of that for you - amplifying the mic and converting the audio signal to digital.