possible problem with my sound card

You must have plugged the microphone into the "headphone" jack rather than the mic jack.  When you plug something into the headphone jack it disables the rear speaker output.  If that is what happened, it sounds like the headphone jack may be damaged.  You can open the case and get access to the front panel mic/headphone assembly.  There is a white plug on the small board, which is the audio connections.  Try reseating the cable to see if that restores the rear speaker output.  Also, look at the headphone jack to see if there is any damage.  You can also try disconnecting the cable to the white plug and see if that restores the rear speaker output. 

I doubt that it's a driver or the SigmaTel audio, since it stopped working when you plugged into the front jack.   That front panel assembly, if it's defective, can be replaced and there was a recent post, here, about that panel (it's used on several Dell models).  Dell spare parts may have the panel or a 3rd party reseller or e-bay may have one - if that's what is needed and finally if it is the I/O panel and you can't get the panel an entire replacement case is probably not that much and then take the I/O panel out of the replacement case (easier than switching everything into the replacement case).

One final thought, plug a headphone into the rear GREEN speaker jack and see if you have sound, just in case it's actually a speaker problem.

I've never used the headphone jack on the front of the pc until recently. And my speakers were fine. All connections are solid. If the rear speaker output is disabled, how do i enable it?  When i boot the pc, i hear the sound of the speakers powering up. If i turn the volume all the way up, you can barely hear noise, but it is very broken up. I'm not going to use the mic/headphone anymore, i just want to restore the sound. What else can i try? Thanks for your input.

As I noted,if the rear speaker output is muted or very low and it's not the Volume Control in the Volume Control Panel, it's probably a problem in the front panel headphone/mic assembly.  Connect a Headphone to the front headphone jack and see if you have good audio volume and then connect the headphone to the rear speaker jack and see if you have good audio volume.   If it's good on the front and bad on the rear it's a mechanical/electronics problem - there is nothing the PC to select front or rear audio out.   

If it's bad on the front panel headphone jack then it's probably the sound module or drivers.  You need to do some troubleshooting to determine what or where the problem is.  Help me help you. 

If it's bad on the front panel, uninstall the SigmaTel sound in the Device Manager then restart the PC and when Windows starts it will detected and reinstall the sound.  This fixes a lot of flakey problems.  To access the Device Manager, hold the Windows key down and press the Pause/Break key.  Right click on the SigmaTel in the Audio section and then Left click on Uninstall (do not uninstall drivers, just the SigmaTel).