Fluidsynth (Qsynth) only sounds when Pulseaudio Volume Control is open (using PipeWire)

I have newly installed Lubuntu, and used the Ubuntu Studio Installer to setup the audio environment. There is a strange thing happening, when I send midi-messages with Python rtmidi package to Fluidsynth:
I can’t hear anything unless I have the Pulseaudio Volume Control opened, then all the sounds happen at once! If I keep the Pulseaudio Volume Control I can hear the results, otherwise same thing happens as I described above!
Can any one help?

(Not sure whether this has to do with the problem) Also using my volume-buttons of the laptop to change the volume level has no effect, I have to adjust it with the mouse.

Release Details:

~$ lsb_release -a

Distributor ID: Ubuntu
Description:    Ubuntu 24.04.1 LTS
Release:        24.04
Codename:       noble