When you say you reinstalled the drivers, and then checked for updates, from whence did you get the drivers and check for updates? If it was from Windows Update or the "Update Drivers" button in the device manager, there's a good chance that's your problem. Usually it takes an event like this for people to realize the folly of downloading drivers from Microsoft.
So, following along this line of reasoning, go to Dell's support site, enter in your service tag, then download the latest audio drivers there.
If you already did that, then what I would do is download a Linux LiveCD like Knoppix. Burn that to a disc, boot from it, and then see if you can get audio. Since this is a completely different OS, and running off a CD/DVD, it helps narrow it down to either a stupid Windows trick or a hardware issue. A 3+ year old laptop should have pretty decent hardware support on Linux, so if you're still not getting any audio odds are the motherboard is bad. However, if you do get audio under Linux, then you know that the issue is with Windows.
I read through an old thread of the same issue & tried everything that was suggested there but I still have no luck in getting audio back on my Inspiron 1525.

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