I want to use the external speakers and the internal speakers so I bought this device, which I think will work. I am hoping this will give me a deeper sound. Not that my speakers are bad they aren't but I am willing to try. At the least I can use it to plug in my unpowered mic which does not work in the rear jack of the iMac.
http://cgi.ebay.com/USB-2-0-to-Mic-Speaker-5-1-Audio-Sound-Card-Adapter-K02_W0QQitemZ280267978957QQihZ018QQcategoryZ3701QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/USB-2-0-to-Mic-Speaker-5-1-Audio-Sound-Card-Adapter-K02_W0QQitemZ280267978957QQihZ018QQcategoryZ3701QQssPageNameZWDVWQQrdZ1QQcmdZViewItem