C/C++ Qt multimedia requirements

Seeing how Qt is supposedly using Gstreamer under Unix, would there be any set up required to use Qt multimedia namespace in FreeBSD? Or is it as simple as install Gstreamer?

I tried to build a small test app with QtMultimedia classes in FreeBSD where there is a working Realtek 877 driver but Qt could not detect any multimedia devices.
The Qt folks suggested to check the Qt plugins. ALSA or PulseAudio seem to be the audio plugins for Unix.
I am confused: what is the role of Gstreamer as Unix audio back-end for Qt versus the role of for instance ALSA as an audio plugin???
KDE is written in Qt, and KDE audio works on this machine, so I am at a loss to understand what is missing for Qt library to work with audio.
Anyhow, what is the process to install ALSA? Is that as simple as pkg install alsa-utils alsa-libs alsa-plugins?