I tried updating the audio driver recommended for my computer (IDT 92HD73C1 Audio, v.6.10.0.6267, A18), but this didn't seem to work, and now I've gone back to a previous restore point to get back to square one.

I can't tell from that if you weren't able to install the IDT driver or if you installed it but got the same error message.

If the former, try downloading one of the IDT files directly from Dell's ftp server.

If the latter, if you get the same symptoms with both the IDT driver and the Windows native driver (High Definition Audio Device) then it is hard to see the drivers as the cause of the issue because it would be rare for both drivers to have the same failure at the same time. Here is how to toggle between the 2 drivers:

1. Open the Device Manager (type devmgmt.msc in the search box and hit Enter) and right click on IDT High Definition Audio Codec.
2. Select to "Update Driver Software".
3. Click on "Browse my computer for driver software".
4. Click "Let me pick from a list of drivers on my computer".
5. Put a check in the box "Show compatible hardware" if not already checked.
6. In the list of devices, click "High Definition Audio".
7. Click "Next".
8. On the Update Driver Warning box, click "Yes" (install the driver).
9. Restart the laptop if prompted. If not prompted, then no need to restart.

To reverse the process, switch the names in steps 1 and 6.