After many hours of reading, researching and debugging, I have solved my audio problem on my inspiron 9300 but think what I did may help many of the others.

I too did all the suggested things like re-installing quickset, audio drivers, etc. to no avail. Finally I looked in the sound and audio devices section under control panel and noticed under the audio tab my default devices were USB audio instead of the SigmaTel C-major audio device. I changed it and viola !!! All is well using the fn keys OR the media control buttons on the front of my laptop.

Hope this works for you .... can't believe Dell Support didn't mention this ANYWHERE !!

That's hours of my life I'm never getting back ....

Good luck !!