HDMI sound output missing from unity-control-center

I have a Dell U3471W monitor with an integrated loudspeaker attached to a Dell T5600 workstation via DP.

"unity-control-center" does not show the monitor as an HDMI/DP output device, while "pavucontrol" does show it. Therefore I am unable to switch back to HDMI/DP sound output from other available outputs: internal workstation speaker when enabled or USB headphone.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: unity 7.4.5+16.04.20180221-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-13.14~16.04.1-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
.proc.driver.nvidia.gpus.0000.03.00.0: Error: [Errno 21] Is a directory: '/proc/driver/nvidia/gpus/0000:03:00.0'
.proc.driver.nvidia.registry: Binary: ""
.proc.driver.nvidia.version:
 NVRM version: NVIDIA UNIX x86_64 Kernel Module 384.130 Wed Mar 21 03:37:26 PDT 2018
 GCC version: gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9)
.proc.driver.nvidia.warnings.fbdev:
 Your system is not currently configured to drive a VGA console
 on the primary VGA device. The NVIDIA Linux graphics driver
 requires the use of a text-mode VGA console. Use of other console
 drivers including, but not limited to, vesafb, may result in
 corruption and stability problems, and is not supported.
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
BootLog: /dev/sda1: clean, 548544/1875968 files, 5987381/7499776 blocks
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Mon Sep 3 14:32:57 2018
DistUpgraded: Fresh install
DistroCodename: xenial
DistroVariant: ubuntu
ExecutablePath: /usr/bin/compiz
GraphicsCard:
 NVIDIA Corporation GF100GL [Quadro 4000] [10de:06dd] (rev a1) (prog-if 00 [VGA controller])
   Subsystem: NVIDIA Corporation GF100GL [Quadro 4000] [10de:0780]
MachineType: Dell Inc. Precision T5600
ProcEnviron:
 PATH=(custom, user)
 SHELL=/bin/bash
 LANG=fr_FR.UTF-8
 LANGUAGE=fr_FR:
 XDG_RUNTIME_DIR=<set>
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-13-generic root=UUID=5aeb2f28-b5ad-4b54-a5eb-dd2ee562586f ro splash noibpb quiet vt.handoff=7
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/21/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A18
dmi.board.asset.tag: IS223528
dmi.board.name: 0GN6JF
dmi.board.vendor: Dell Inc.
dmi.board.version: A01
dmi.chassis.asset.tag: IS223528
dmi.chassis.type: 7
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA18:bd06/21/2018:svnDellInc.:pnPrecisionT5600:pvr01:rvnDellInc.:rn0GN6JF:rvrA01:cvnDellInc.:ct7:cvr:
dmi.product.name: Precision T5600
dmi.product.version: 01
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.12.3+16.04.20180221-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.91-2~16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 18.0.5-0ubuntu0~16.04.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 18.0.5-0ubuntu0~16.04.1
version.nvidia-graphics-drivers: nvidia-graphics-drivers-* N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.18.4-0ubuntu0.8
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1.2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2
xserver.bootTime: Mon Sep 3 14:07:49 2018
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:

xserver.version: 2:1.18.4-0ubuntu0.8