Finally hunted it down, the missing package is one of the gstreamer-plugins. More specifically gst-plugins-good was missing on my laptop although in your situation I recommend installing the whole shebang sudo pacman -S gst-plugin-pipewire gst-plugins-bad gst-plugins-bad-libs gst-plugins-base gst-plugins-base-libs gst-plugins-good gst-plugins-ugly gstreamer phonon-qt5-gstreamer
Edit: You can omit the pipewire one, if you are not using pipewire.
Edit2: Don’t forget to close and then reopen the system settings app after you install it.