connecting a microphone to an iMac?

You probably have one of two problems:

1. Your headset's has an un-amplified mic (no battery). The audio input on a Mac is a line level input and will not work with this type of mic.

2. Assuming your headset has an amplified or usb mic, you may need to go to System Preferences > Sound and select your headsets mic and not the iMacs mic as the input source.

Are you using an adapter on the male plug(s)? Remember that mono has one black "line" and stereo has two, while a combination stereo plus microphone will have three. These lines are like slim black rings around the part of the plug you insert. Things won't work if you put the wrong adapter on.

You are saying you have a 7.1 channel headset with a microphone? It would be very helpful if you could tell us what kind of connector(s) it uses. USB and "soundcard" are two different things. There are quite a lot of excellent outboard USB audio interfaces for professional use (nothing to do with soundcards or installing a card, they just plug in to the USB port) but it's not clear that you need anything like that. Please give us some more information about what you are working with! Thanks ~~