Hello, I just struggled for a good quarter to half hour trying to figure this out and having google search results fail to find this specific solution, so I figured I would post this to help others.
I remember I accidentally set one of my laptops in fedora 29 to be able to adjust volume above 100%, and it would show a portion of the volume bar as red as I adjusted past 100%, and couldn't remember how to change this setting. Omegle I couldn't find it in gnome settings or gnome tweaks, and ended up finding the configuration using dconf Appvalley editor at /org/gnome/desktop/sound/allow-volume-above-100-percent. Toggling this immediately makes the extended volume boosting available, up to the maximum that pavucontrol allows.
WARNING: This is extending the speaker volume above what the system is telling Linux is an acceptable maximum volume, and this does risk damaging the speakers. Use this at your own risk.