I had a similar issue with dual-booted Ubuntu 18.04 + Windows 10.
I had very quiet sound on laptop speakers on Linux, but Windows 10 was fine. Using headphones under linux also worked fine. None of the above software solutions worked for me, and the 200% volume setting was still quiet and had very poor sound quality. Rebooting the machine had no effect.
After a couple of days, the issue suddenly fixed itself when I disconnected my USB headphones whilst playing audio.
My guess is that there was some hardware/software switch for detecting if the headphones where plugged in that had not been properly reset.
So try playing some audio, then plug in a set of headphones and yank them out again. Possibly try this a few times and see if this helps.