The default configuration is when the headphones are unplugged it disables the headphone volume.
In your case it appears the headphone volume controls the internal laptop speaker volume.
A potential workaround is to reconfigure the headphone volume so it isn't disabled when the headphones are unplugged.
nano /usr/share/pulseaudio/alsa-mixer/paths/analog-output-speaker.conf
Change
[Element Headphone]
switch = off
volume = off
to
[Element Headphone]
switch = off
volume = merge
robbie got a reaction from cardinal in Noob audio help, no sound on speakers with ALSA or Pulse