Ubuntu: force sound to a Bluetooth output

On Ubuntu 14.04 LTS, a critical and unpatched bug has been reported.
When a Bluetooth device is paired, it is impossible to force the system to play sound through it.
The workaround is to force the output in shell:

sudo pactl list cards short
0	alsa_card.pci-0000_00_1b.0	module-alsa-card.c
4	bluez_card.FC_58_FA_89_91_2C	module-bluetooth-device.c

sudo pactl set-card-profile 4 a2dp

Version context

This procedure describes the software and the environment available at the time of its publication. The packages, interfaces, and repositories mentioned may have changed or disappeared; first check the system version and its support level.

cat /etc/os-release
uname -r

Work on a backup or a snapshot, note every modified file, and prepare for rollback. For a current workstation or server, prefer signed packages from officially supported repositories.

Reference: official Ubuntu lifecycle.