Software dmixing not working between logged user and mpd

[nojhan]

After having installed mpd 0.19.12-1 and succesfully played music with it, it remains impossible to play sound as the logged user, except if manually using the dmix pluggin.

In othe words, when nothing is playing, this does not output any sound, but does not produce error:

$ aplay /usr/share/sounds/alsa/Front_Center.wav
Lecture WAVE '/usr/share/sounds/alsa/Front_Center.wav' : Signed 16 bit Little Endian, Fréquence 48000 Hz, Mono

But this works:
$ aplay -D plug:dmix /usr/share/sounds/alsa/Front_Center.wav
Lecture WAVE '/usr/share/sounds/alsa/Front_Center.wav' : Signed 16 bit Little Endian, Fréquence 48000 Hz, Mono

If mpd is told to play music, then the first command does not output any sound, but returns no error:

$ aplay /usr/share/sounds/alsa/Front_Center.wav
Lecture WAVE '/usr/share/sounds/alsa/Front_Center.wav' : Signed 16 bit Little Endian, Fréquence 48000 Hz, Mono

BUT, the second command now returns an error:

$ aplay -D plug:dmix /usr/share/sounds/alsa/Front_Center.wav
ALSA lib pcm_dmix.c:1029:(snd_pcm_dmix_open) unable to open slave
aplay: main:722: erreur à l'ouverture audio: Périphérique ou ressource occupé

The expected default behaviour is that software dmixing should work for the logged user, while playing mpd.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: alsa-base 1.0.25+dfsg-0ubuntu5
ProcVersionSignature: Ubuntu 4.4.0-36.55-generic 4.4.16
Uname: Linux 4.4.0-36-generic i686
NonfreeKernelModules: nvidia_uvm nvidia
ApportVersion: 2.20.1-0ubuntu2.4
Architecture: i386
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: nojhan 3406 F.... pulseaudio
CurrentDesktop: GNOME-Flashback:Unity
Date: Sun Jan 8 17:59:50 2017
InstallationDate: Installed on 2009-01-01 (2929 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build i386 LIVE Binary 20120426-12:41
PackageArchitecture: all
SourcePackage: alsa-driver
Symptom: audio
Symptom_AudioUsers: pulse
Symptom_Card: Audio interne - HDA NVidia
Symptom_DevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: nojhan 3406 F.... pulseaudio
Symptom_Jack: Green Line Out, Rear
Symptom_Type: Sound works for some users but not for others
Title: [HDA-Intel - HDA NVidia, playback] sound not working for all users
UpgradeStatus: Upgraded to xenial on 2016-09-03 (126 days ago)
dmi.bios.date: 11/17/2009
dmi.bios.vendor: Phoenix Technologies, LTD
dmi.bios.version: ASUS P5N32-E SLI PLUS ACPI BIOS Revision 1502
dmi.board.name: P5N32-E SLI PLUS
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: 1.XX
dmi.chassis.asset.tag: 123456789000
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnPhoenixTechnologies,LTD:bvrASUSP5N32-ESLIPLUSACPIBIOSRevision1502:bd11/17/2009:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP5N32-ESLIPLUS:rvr1.XX:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer