Ubuntu how can I send my audio output, to the mobile which is connected using bluetooth?

If you use Blueman instead of gnome bluetooth manager you can set uo your phone as sound output.

You can find Blueman in Software Center

I also tried to do this in the past, searched for answers and the basic answer for this question is that you can't do it, because the android bluetooth stack lacks support for A2DP sink capabilities.

Then I found a great answer at stackoverflow which says that it might be possible to do it, but it's hard to do. The answer and the details are here. For the solution presented in that answer you will need:

• a rooted device.
• Good programming skills/experience. (Or someone who does it for you)
• Android version older than 4.2, because in 4.2 Android switched from the BlueZ stack to Broadcom's Bluedroid.

But I don't know whether the solution presented in that answer really works, because I never tried it. Maybe in the future Google will change the default bluetooth stack to support the A2DP sink profile or some modders make a custom ROM with support for it, but until that day this is the only way.