Play sound through two or more outputs/devices

With paprefs you have access to a virtual output device that enables simultaneous output to all attached sound cards and devices:

sudo apt install paprefs

Then in the terminal run paprefs, select Simultaneous Output tab, and check Add virtual output for simultaneous output on all local sounds cards.

The additionally created audio output device for simultaneous output may be selected in the Output tab from the PulseAudio Sound Preferences menu (pavucontrol):

In this example it is shown for an HDMI-device, but, as soon as your Bluetooth device is recognized, it will also be available for simultaneous output.

The changes may need a restart of PulseAudio to take effect, either by logging out and back in to your session or by running pulseaudio -k in a terminal.

In case paprefs does not do the job or if you prefer to have paprefs not installed, use this command from the command line:

pactl load-module module-combine-sink

To unload the module from the command line and reset PulseAudio to defaults, just restart PulseAudio with:

pulseaudio -k

or issue:

pactl unload-module module-combine-sink

Ubuntu

I've just confirmed this solution still works on Ubuntu 14.04 LTS.

1. Install: Open terminal and type sudo apt install paprefs go to the tab exactly as per the picture above and select the option.
2. Initial Run:
   1. Remaining in terminal, type pulseaudio -k to kill and restart pulseaudio (this way with current systemd pulseaudio user service).
   2. Then go to your sound settings and you will see the option to output to multiple sound devices.
   3. Props to whoever wrote paprefs it's a brilliant little piece of software I would actually like to see included in Ubuntu without requiring additional installation.

Apple Mac OS X

A similar solution is available via an included piece of software and whats so great about pulseaudio is that the sound seems to be perfectly in-sync from both outputs so it must be adjusting for the lag as well which is why its so impressive; otherwise we would be hearing a slightly delayed version from one output and another.

In Kubuntu 18.04, Plasma 5.12 paprefs is not needed, as a similar setting is already there:

A new output option should become available after reboot, called “Simultaneous output”.

Or in pavucontrol:

In 18.10 with Plasma 5.13.5 that “Simultaneous output” option has been removed, so paprefs is needed.

I could not get Takkat's proposal work out of the box in Debian 8.7, although I restarted the system. I assume you have completed Takkat's proposal in installing paprefs. Extension on Takkat's answer which works based on Arch Linux wiki where keep analog input and Pulse calls that "duplex"

# /etc/pulse/default.pa
# http://unix.stackexchange.com/a/180374/16920
load-module module-alsa-sink device=hdmi:0
load-module module-combine-sink sink_name=combined
set-default-sink combined

Then restart pulseaudio:

pulseaudio -k

Because LeonidMew was asking about 18.04 (I'm using 18.04.2), here's my version.

The GUI paprefs tried to combine one HDMI with one analog output, instead of both hdmi. So I edited the file /etc/pulse/default.pa as described by Léo Léopold Hertz

$ gedit /etc/pulse/default.pa  # make changes as in method a or b below
$ pulseaudio -k # then restart pulseaudio

Both the following methods work on their own, so pick one (make sure to add these lines to the top of the file, I put it right after .fail ! Otherwise it doesn't work).

Method A

load-module module-alsa-sink device=hw:0,0
load-module module-combine-sink sink_name=combined

Method B

load-module module-alsa-sink device=hw:1,3 sink_name=hdmi
load-module module-alsa-sink device=hw:1,7 sink_name=hdmi2
load-module module-combine-sink sink_name=combined slaves=hdmi,hdmi2
set-default-sink hdmi-combined

Reference

Method A

For reference, the hw:0,0 comes from aplay -l

$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC892 Analog [ALC892 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: ALC892 Digital [ALC892 Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 8: HDMI 2 [HDMI 2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 9: HDMI 3 [HDMI 3]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Method B

And the 1:7 and 1:3 comes from

$ pacmd list-sinks | grep -e 'name:' -e 'alsa.device ' -e 'alsa.subdevice '
    name: <alsa_output.pci-0000_00_1b.0.iec958-stereo>
        alsa.subdevice = "0"
        alsa.device = "1"
    name: <alsa_output.pci-0000_01_00.1.hdmi-stereo-extra1>
        alsa.subdevice = "0"
        alsa.device = "7"

On the GNOME "sound settings" I set the other HDMI from 2nd monitor as output, and then re-ran the command

$ pacmd list-sinks | grep -e 'name:' -e 'alsa.device ' -e 'alsa.subdevice '
    name: <alsa_output.pci-0000_00_1b.0.iec958-stereo>
        alsa.subdevice = "0"
        alsa.device = "1"
    name: <alsa_output.pci-0000_01_00.1.hdmi-stereo>
        alsa.subdevice = "0"
        alsa.device = "3"

To verify these numbers are correct, I ran

$ aplay -D plughw:1,3 /usr/share/sounds/alsa/Front_Right.wav
$ aplay -D plughw:1,7 /usr/share/sounds/alsa/Front_Right.wav

which sounded on their respective monitors.

To complement the excellent answer from Takkat, I found the default name given to the new device was excessively long and distorted the Sound Settings dialog. In order to shorten that name, I had to additionally execute the following command:

gconftool --set --type string /system/pulseaudio/modules/combine/args0 sink_properties=device.description=Combined