Problem:

You want to connect a bluetooth HiFi headset/headphone and enjoy high quality
audio using the a2dp_sink with pulseaudio, but you get only low quality,
handset quality audio, because a2dp_sink is not available.

How to reproduce:

Connect your bt HiFi headphone, while module hidp is NOT loaded and invoke 
"pactl list" 
from pulseaudio utils, you may see this line:

a2dp_sink: High Fidelity Playback (A2DP-target) (sinks: 1, sources: 0,
priority: 40, available: no)

==> "available: no"  this tells you, A2DP is not available for your bt
headphone


Root of the problem:

In some cases the hidp module does not get automatically loaded, to me this
seems the case with laptops that have integrated bluetooth. The module
bluetooth gets loaded, but not hidp. When using an external USB bluetooth
dongle, manually connected, hidp *may* get loaded.


Solution:

It must be sure, that module hidp gets loaded after module bluetooth was loaded
Without module hidp there will be no A2DP audio over bluetooth.

You may consider to add this line to /etc/modprobe.d/50-bluetooth.conf 

softdep bluetooth post: hidp



Checking the problem is solved by suggested solution:


After hidp was loaded, connect your HiFi bt headset/headphone again and invoke
"pactl list"
you should then this:

a2dp_sink: High Fidelity Playback (A2DP-Ziel) (sinks: 1, sources: 0, priority:
40, available: yes)

==> "available: yes"  is what you want, A2DP audio now works