Previous driver was 22.214.171.12411 dated 12 Aug 2016 and the update is 126.96.36.19917 dated 19 Dec 2016, listed as Dell revision A07. Download Audio_Driver_RT1XX_WN32_188.8.131.5217_A07.EXE
Worth noting the Dell Command Update utility (v2.3.1) for checking if updates are available, didn't list this particular audio driver update for download. We only spotted it when manually checking the Dell support website.
(Just for completeness - we'd also loaded BIOS 1.5.10 dated 9 May 2017 at the same time, but don't think that was relevant. There was mention of a LAN driver possibly causing audio interference, this system was already on the latest Dell download 20.4 A01 with driver 184.108.40.206 dated 5 Oct 2015 for Intel l219-LM. Intel have a newer release 22.3 ProWinx64 with driver 220.127.116.11 dated 19 Mar 2017. Hyper-V and VMware virtual adapters were present)