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