Q: How do I connect my external speakers and input audio on the new iMac's?

There is no audio input on the new iMacs.

But a regular USB audio solution would probably work

So typical of backward thinking to distress users who need solutions when no problem existed before (ie. sound quality and 8th inch audio jacks.  I'm sorry to have taken apple product for granted and want the one ripped off from me back.  Yes, nice touches on prehistoric faux pas of past machines, for which there was little excuse in the first place.  Putting up with changes that are not identified and make huge differences to people interested in music aside from different colored loops I'd have hoped were prohibitive (like the many sales technique across the board.  Sorry to put my observation in subdued terms.  Thanks.

http://support.apple.com/kb/HT4587

This should fix you up.