I've seen this with laptops that didn't get the usual 6 month canned air or it's time to go inside to do the deep cleaning and replace heat sink compound. It's certainly old enough for the 100,000 mile maintenance job.
There are also other reasons such as toxic combinations of software and accepting drivers from Microsoft.
But let's say the audio is indeed dead. Why would I suffer the expense of a new motherboard over this 2 buck fix -> http://www.amazon.com/Virtual-5-1-surround-External-Sound-Card/dp/B000N35A0Y/
I carry one of those in my PC ready of PC First Aid kit.
My buddy, my wonderful laptop that has served me well for the last five years without a single hiccup, has now developed sound problems. Music skips. Calls with Google Call Phone, normally so clear that no one knows I'm calling on my computer, are now as bad as Skype. People can't hear me. Scratchy background noise. Frequent cut-offs.
I did some research only to find that Sigmatel is defunct. Any thoughts on either repair or replacement, if it won't cost more than the laptop?