I purchased a USB external sound card as I was unable to get my external microphone to work with my iMAC. When I received it today the Microphone works and an added bonus is that it simulates 5.1 sound on my stereo speakers giving them a much better sound. I spent a total of $5.00 for this little gadget. Below is a link to what I bought. One thing I was curious about is if you can select both internal and external speakers and I don't think it could be done nor do I think there is a reason to do so.
http://cgi.ebay.com/USB-2-0-Mic-Speaker-5-1-Audio-Sound-Card-ADAPTER-a-9340_W0QQitemZ160302628566QQihZ006QQcategoryZ118154QQcmdZViewItemQQ_trksidZp1742.m153.l1262
http://cgi.ebay.com/USB-2-0-Mic-Speaker-5-1-Audio-Sound-Card-ADAPTER-a-9340_W0QQitemZ160302628566QQihZ006QQcategoryZ118154QQcmdZViewItemQQ_trksidZp1742.m153.l1262