Lost hardware volume controls after upgrade to 16.04 HWE Stack

I am running Ubuntu 14.04 on a Dell XPS 12 laptop. After upgrading to the 16.04 Xenial HWE Stack, the following buttons on the casing stopped working:

- volume controls
- ‘windows button’ below the screen (this used to bring up the Dash)

I’m not bothered about the second point. But is there any way to bring back the volume controls?
Volume control via the keyboard (fn+f11/f12/f13) is still working fine.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-4.4.0-36-generic 4.4.0-36.55~14.04.1
ProcVersionSignature: Ubuntu 4.4.0-36.55~14.04.1-generic 4.4.16
Uname: Linux 4.4.0-36-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Sep 7 21:06:26 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2016-01-09 (241 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
SourcePackage: linux-lts-xenial
UpgradeStatus: No upgrade log present (probably fresh install)