How do you setup a microphone for audacity on imac?
Depends on the mic. Mics can use the following connectors: 3.5mm, 1/4" connector; XLR, USB. Step 1 is to take care of the connection. The 3.5mm connector on the back of my iMac is for headphones, so plugging the mic into that won't work. Not a bad thing, it just is - The iMac's built-in mic is good, so it really does not matter... unless the mic needs to be further away from the iMac than the built-in can deal with. Years ago I got an Onkyo MSE-U33HB USB audio processor with a USB computer connection. It has a 3.5mm audio input that works great. If I had to do that connection today, I'd probably get a Focusrite Scarlett, Mackie Onyx or similar USB audio interface. If the mic uses a 1/4" connector, again, use the USB interface... like the Focusrite or Mackie gear. If you need to connect more than 1 mic, then use a proper audio mixer (I use an 8 channel Alesis Multimix USB mixer - it can handle multiple mics and any of the analog connection methods (3.5mm, 1/4 inch, XLR). If the mic connects using USB, then plug it into one of the USB ports on the back of the iMac. If you want to use more than 1 USB mic, then you have a problem - this is not a Mac-specific issue - same problem exists with Windows computers. The USB interface should come with some software - it may have something the Mac can use - or not. If not, this does not mean the interface can't be used, it will "just work". In any case, once the connection is made, go to the iMac's System Preferences: Sound. The USB interface should cause the device (USB interface, USB mic) to appear in the box beneath "Select a device for sound input"... and click once on it to select it and close the System Preferences windows. Then, when Audacity is launched, it should use the audio input you just selected. If not, then use Audacity's audio input selection.
These should be compatible with Apple, but I'm not sure about iMac specifically: https://www.bestproducts.com/tech/gadgets/g1422/external-sound-cards-for-pc-or-mac/ Several have microphone inputs, but I suppose your microphone may have an XLR output. https://www.parts-express.com/Data/Default/Images/Catalog/Original/092-013_HR_0.jpg