You may have a damaged or disconnected audio board. Use [guide|1059|this guide] to open the computer and get to the board. Check the ribbon cable coming from the interconnect board is connected to it. If the cables aren't the problem, you may need to replace the board itself. [http://www.ebay.com/itm/NEW-AUDIO-BOARD-PN-922-8803-FOR-MAC-MINI-2009-A1283-/110877770724?pt=LH_DefaultDomain_0&hash=item19d0d483e4|Here is one for sale on eBay], as iFixit does not sell them.