You can use an external microphone. You might want to check to see if it's just disconnected. See steps 6 & 7 of this guide: [guide|896]
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That failing, I believe this is the cable you need: http://www.gainsaver.com/Catalog/Detail.aspx?&CCode=2767^Apple+Parts&CCode=2778^iMacs&CCode=2860^iMac+%28Early+2006+17-inch%29&cICode=130268