Microsoft have included the IDT High Definition Audio CODEC as an optional update within Windows Update. Installing it killed my audio as it has done for a number of other users.
I’d recommend not installing it for now.
(I ended up fixing it by uninstalling it and reinstalling the Dell audio drivers from scratch. Other options would be using System Restore to go back to before you installed it.)
I’m kind of disappointed because Windows Update has always been so amazingly solid for me in the past.