System volume too low on Ubuntu 18.04

As a temporary solution, you can manually set it higher than 100% from the command line with:

pactl set-sink-volume 0 150%

If the command is not found, you need to install:

sudo apt install pulseaudio-utils

The 0 there is the index of the sound card sink you want to use. You can determine it with:

pacmd list-sinks | grep -e 'name:' -e 'index' 

The 150% is the percentage of volume you want. Start with 150% and work from there slowly. You dont want to blow your laptop speakers with clipped audio

Unverified, because I don't have the hardware anymore.

Most RealTek audio chips have a dedicated headphone amplifier that needs to be enabled if you want to connect headphones to it. By default, it is bypassed, as it introduces a bit of noise, and the amplifier is unnecessary if you connect another amplifier anyway.

In the ALSA sound system, there would be a switch in alsamixer for the amplifier, shown as a mixer channel with no slider, just a mute button, and pressing m to mute/unmute would activate and deactivate the amplifier.

In PulseAudio, I would expect this to show up in pavucontrol, either as a separate port (so it can be selected on the "Output Devices" tab), or as a device profile (which you would select on the "Configuration" tab).

You can change the option in "Setting > Sound > Over-amplification>>on" if you want higher volume.