Use Ubuntu computer as bluetooth speaker for an Adroid device

apt-get install bluemon
blueman-manager
connect to a device as A2DP Source (Receive Audio)
I had to to run this first to make it work, though:
pactl load-module module-bluetooth-discover

Sources:
https://crcok.wordpress.com/2014/10/13/bluetooth-device-added-successfully-but-failed-to-connect/
https://ceworkbench.wordpress.com/2011/03/21/configuring-linux-as-an-a2dp-audio-sink/