Q: External audio not connecting (thunderbolt/airplay/bluetooth)

Hi,

I'm running OS/X El Capitan 10.11.4 on MacBook Pro (Retina, 15-inch, Mid 2014).

For some time now - I cannot connect my audio to any external audio outputs/interfaces

- when I change my sound output using alt+volume icon at the top menu, nothing happens (when I come back to this menu output remains in "internal speakers")

- When I select it in "sound" under system preferences it is being selected, but sound is still coming from internal speakers.

The external devices include

- Airplay to ATV2 - it is worth mentioning that is see "airplay: off" in the airplay menu

- Bluetooth audio - pairs and connects fine, but the same happens for audio

- Focusrite clarett 4pre thunderbolt audio interface. I installed the required software & drivers prior to connecting the device. When I turn the device on - I cannot see it as a sound output in the sound menu, though it is appearing under the thunderbolt section in "System Information".

- I do have a firewire audio interface (steinberg MR816), which was installed long before the problem occurs, and this device connects just fine.

I have already tried:

- Deleting audio & bluetooth & airplay related .perf files in /Library and ~/Library, and restarting

- Removing all apps from "User & Groups" login items to improve airplay connectivity.

- Resetting PRAM

- Creating a new user with "clean" settings

- I might have messed with other settings which I don't currently remember.

I could not find anywhere to tell me how to reset the mac's audio settings completely, and I would really like to avoid formatting & reinstalling the mac.

I would be extremely helpful for any help.

Thanks