Probably after some system updates, the sound of the internal microphone seems not to be passed to applications any more.
In GNOME Sound Recorder and other applications, it seems that the input of the internal microphone is 0. However, in GNOME Settings and Volume Control (pavucontrol) I do see that the internal microphone detects sounds, because the input graphs for it are moving.
In Bugzilla there is this bug with more information: 2089012 – Microphone does not work >= 5.17.7. This bug is now categorized as a kernel bug, which is very probably not the case for my issue, as I do see that the internal microphone works in GNOME Settings and Volume Control, but the input sound is not passed to the applications.
Do you have any suggestions on how I could solve or debug this?