Ubuntu 18.04 bluetooth low volume

Installing pavucontrol solved the problem for me on my Thinkpad T420, Ubuntu 18.04, and a Bose Soundlink Revolve+.

Run:

sudo apt install pavucontrol
pavucontrol

Connect your Bluetooth device to your laptop and start feeding it. Then you should be able to adjust the maximum sound level inside pavucontrol. The volume keys on the laptop work as before, except the maximum is now higher. The settings appear to survive application restarts.

I have the same problem with Sony WF-1000XM3. They are quite silent on Ubuntu 20.04. Latest Sony firmware update added ability to control volume directly on earphones and it helps. Unfortunately you lose possibility to use other functions on one of earphones.

Bumping @Jeff Ward's comment to an answer for those who found this question because they had low volume on Sony WF-1000XM3/Sony WH-1000XM2/Bose NC 700 bluetooth headphones:

Same here on the Bose NC 700 -- a firmware update set the volume super low. The solution was turning the volume up on the headphones themselves (via touch - swipe up on the right-front ear cup.)

In my case, adjusting the volume on the headphones themselves was the only solution - no computer setting was able to increase the volume.